The foam I was referring to was...
...the expanded polyethylene stuff that dishes are wrapped in, only purchased in six inch thick blocks. You would glue (with the 3M super spray glue) and pin (with wooden dowels through sections not housing a horn part) the blocks of foam together, and then carve out the cavities with a "hot wire" (a length of nichrome wire on a wooden stick, attached to a model railroad transformer. Once the cavity is to size, you then send it off to the box maker, where it is covered and installed in the case.
I forget the prices quoted, but I know that the most expensive element of the whole project was the raw blocks of foam, with prices up in the fifty dollar range each. The hot wire cost ten bucks or so to put together, the transformer could be had for another ten, and the glue and dowels for another ten. The finished case cost from the box maker was less than all of the rest put together.
My bass clarinet case is dying, I can't get a replacement that fits (since Selmer no longer has them and the aftermarket stuff is too cheap and too "universal" to work) and I would love to replace it with a combination bass clarinet/clarinet case made to my spec (storage, latches that work, a handle that doesn't look like it came from a 1920's suitcase), but the blocks of the polyethylene foam are a) expensive and b) hard to find.