Thanks Gandalfe for the tip. npi I ended up getting a couple each for my Bb bass clarinet and Bb clarinet from across the ocean. My friend wanted one as soon as he saw it so he could put it on to keep the tip wet while he switched to another instrument.
I experimented with making the short caps in TPU on a 3D printer. I posted my OpenScad code on Thingiverse. They looked promising at first but I found the concept did not stay well on some mouthpiece shapes. It probably works better with the rubber used in the injection molded caps by Silverstein and others.
I moved on to making more traditional mouthpiece caps out of PLA. To get a great fit, I need to have the mouthpiece for test fits. I also recently found that the wall thickness needs to be 1.6 mm thick (4 layers). At 1.2 mm thick, the PLA material changes shapes while stored on the mouthpiece and the fit gets loose. Shorter expansion slits in the design also seem better.
OpenScad code to make custom sized mouthpiece caps. They have a long slit that can be changed to a shorter one for less spring. I find they get looser once stored on the mouthpiece. I have included an Excel spreadsheet that contains several parameters I sized for various mouthpiece and...
www.thingiverse.com
The long slit design does work well on Rovner caps. Check them out too.
This was my source. I suggest getting the larger size as you can always trim a bit and my theory is that the larger size doesn't squeeze the reed till closer to the base where it is more solid.
Somewhat more seriously, I have bought a couple things from AliExpress and didn't have a problem, other than it took a long time to ship. I wouldn't recommend spending much there, particularly on instruments, but less than $5 to try 'em out? I'd be OK with that.
Parametric mouthpiece cap design that can be modified with OpenSCAD program file. The values in the program are for a no name plastic tenor sax mouthpiece I have been using. Measure your own mouthpiece with calipers and plug in the values I suggest in the program. I tried thicker walls at...