My son wanted a plastic mouthpiece and we are using this one to try out different internal shapes to see how they play. This is his favorite shape so far and it is loosely based on the Olds 3 mouthpiece, in that it has a shallow cup and is slightly narrow. I use food safe PETG and sand the rim while spinning it in a drill to get it as smooth as possible. This is far from perfect but it produces a functional mouthpiece when you get done, hope it works for you.
Your favorite Food safe filament
I had trouble getting this to stick on my bed, so I've started printing it with a raft, but I would prefer not to, and ymmv.