When we moved, I blundered into my carefully preserved notes on the custom case box manufacturer up in WI. I put this all together many years ago, when considering putting my own bass clarinet case together (as my current one was disintegrating, slowly but surely).
The principle involved was that you made the guts of the case, and then they would build the box to fit around it. The guts would be made of polyethylene foam, carved out with a hot wire arrangment, then covered with the fabric, and then the whole thing shipped up to them with a couple of extra yards of the fabric. Then, they would make the case box, top cushion and interior, and ship it all back to you.
Those of us with weird equipment combinations (like full Boehm clarinets and extended range basses) could have whatever we wanted (at a price, to be sure). Thoughts of a combination bass/soprano case, or a Traypac for a baritone, quickly ran through my mind, and I drew up the minimal dimensions for such cases and did estimates on the amount of foam need and the hot wire equipment (basically, a model train transformer attached to a U shaped loop of nichrome wire on a wooden handle) needed to make it work.
(My drawing of my bass clarinet components is a hoot to see. The bell and joints are detailed as hell, but the neck looks like some sort of distorted serpent...)
Then I moved away from the Saint Louis area, lost the information that I had accumulated (this being in the pre-internet days), and the plastic foam distributor went out of business. So much for that.
I still have the same bass clarinet case, repaired many times and looking like a piece of luggage from the Indiana Jones theatrical show at WDW. These days, it's someone's else's (probably my son's) problem...
I have found it rather easy to purchase a plastic (on in some cases aluminum) case filled with foam which I then cut to fit what I need (there is an option to get a softer foam that encases the instrument without bending the keys more adaptive to smaller diameter instruments like flute).
Here's an example:my Selmer 9* bass case was wood with a hard plastic interior that was falling apart and I didn't like the way the weight of the upper joint rested on the octave key mechanism. The platsic case and hard foam cost about $60; I used an exacto knife to cut out the foam to fit the horn. The weight was a couple of pounds..so it was quite light and easy to carry longer distances when parking is not easily available. But the foam was hard enough to protect the horn very well (in the attached pic, I had not yet cut out the square box bottom right to hold the neck, mouthpiece swab and reeds. The case was one of a 30 or 40 various sizes that could be cut to fit any number of instrument configurations.
Is that one of those "Pick and Pluck" cases, where the foam is pre-scored in little square columns, and you just remove what you need? (I ask this because of the square-ness of the openings in the case lid, which could have stemmed either from a careful cutting job on your part, or pre-scored foam on their part.)
If so, I'd not trust it, as the weight of a large wood instrument is too way much for the foam in that instance. I've tried the system for some relatively heavy sound equipment, and the weight of the components combined with the vibration of motor travel was all that it took to split the case foam apart. It didn't happen at first, but after a year of hauling things about, it slowly but surely took the foam apart.
Heavy joints (like either of the two in the case of a wooden bass) are about as bad as sound junk. And, unlike sound junk (which is meant to take the abuse that vocalist routinely dish out - I almost killed one vocalist who dropped his cordless mike on the stage (a la rock musician style) after a particularly strong performance, but he assured me that Shure had taken that into account - he also didn't offer to replace the dented microphone screen that he caused), those keys and touch pieces on the bass are a lot more fragile.
If it is cut from the solid, then I would modify my statement to the effect that, as long as certain areas are cut clear (so that the keys are not restrained in those instances), the foam cavity approach will work just fine. The trick is to study the case as designed (allowing for damage to same, and figuring out what is missing), and then design the holes in the new case accordingly.
And, even then things can go wrong. A story covering a long-standing problem:
I had troubles with my low F/clarinet C key work on my bass for thirty years. I would treat the thing like an english horn, carefully storing the joints each and every time, and guarding it against any contact when in use or on a stand. Yet, it would invariably go out after a week or two.
One time, I had it adjusted and loaded in the case by Marvin at Saint Louis Woodwind and Brasswind, and then drove to a job in Kansas City. After carefully removing the horn from the case and putting it together, bingo! there was a leak in the F/C key work. It was at that point that I first suspected the case, and not careless handling on my part. (I would never suspect Marvin of careless anything save his far right wing politics.)
After finishing the job there in Kansas, I took the time to study the case more closely, and then learned that there was a small anomaly in the way that the lower joint fit into the casework. A combination of a loose cushion in the top with a contour in the Plywood cutout in the lower half allowed the joint to (when the case was "vertical") to drop "above" the Plywood portion of the case. Then, when the case was placed "horizontal", the keys (key cups) on the lower section would "hang up" on the Plywood, rather than be carefully contained in the hanging fabric below the cutout.
Rinse and repeat a few times, combined with vibration and "un-cushioned" hits in the trunk of a car, and there's your slight bending that caused the leaks. But, if yo didn't look carefully at the case, you would never have noticed it.
Upon return to the Gateway City, we collectively disassembled the case structure and found - surprise, surprise! - that whoever did the cutout work on the Plywood at Selmer's case fabrication facility had not cut to the lines inscribed on the piece, leaving a "lip" that did the catching and holding. Thanks, Selmer...
Once the "lip" was carefully jig sawed off and the fabric (thirty years old fabric, mind you) was rehung, no more problem.
Obviously, Selmer's cases are not manufactured to the same high standards as their instruments. After this occurred, I had a number of changes made to the cases for my horns, and in each case (pun intended), the modifications corrected long standing issues with the instruments concerned. In one case (for my Puerto Rican full Boehm horn), a case had a blown plastic mould holding the joints, and the plastic had failed, leaving the long levers on the lower joint bearing against a protrusion. Not as critical as the bass problem above, but still needing to be addressed.
The best cases that I've ever had have been the Yamaha baritone cases. I've owned two, the original one with the wheels, and the replacement purchased through eBay (from someone who dumped the case (which is heavy as hell) to use a gig bag (dumb move, at least in my opinion). I've had a horn dropped off of the back of a truck in a Yamaha case, and the horn sailed through the ordeal without a glitch. Hard to believe, but true.
Thank you for compliment on the careful cutting on my part!
There are two forms of foam lining: solid polyethylene, which compresses somewhat, but not very much, and can be cut with a knife, easily, taking out solid blocks of foam. The second type is charcoal ester foam. This foam is softer and more suited to very light weight instruments (i use this type on my traditional Irish flutes and piccolos). I believe that is the pick and pack to which you refer...again, fine for lighter instruments.
My case experience is similar to your English Horn experience...the design and execution was awful....though it was my left hand C# going out of whack (I was playing "New Orleans" when I found this problem with a rather pronounced squawk).
One company will precut the foam for you. I would not recommend for an oboe or english horn...but it suits heavy instruments quite well....and is quite easy to carry.