I have made diaphragms for these with this material from McMaster-Carr: 8969K531 “Ultra-Strength Oil-Resistant Buna-N Rubber, Sheet, 12″ x 12″, 1/64″ Thick” $5.49.
I can’t claim years of testing but it seems to hold up.
I went to a meet a few weeks ago and saw how the diaphragm was constructed.
trex014 was spot on
trex014 solution sounds promising but I’m not going to try it.