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.