You can see how the inner-tube was cut to match the
diameter of the mouthpiece ends. Stiching and silicone
sealant seems to work just fine, although I made a more
shallow curve in the cut of the TP3000, because the point
where the seam meets the mouthpiece takes a lot of flexing,
and I had a couple of small leaks there until I really went
nuts with reinforcing rubber.