Melodica-Me, thanks for the write up. I’m about to replace the bladder on my basso and this helps a lot.
There’s a wide range of neoprene sheets available from McMaster-Carr. Several options for hardness, strength, food grade, thickness seems to start at 1/64 inch.
You report on the another thread thread that playability improved after your replacement. Perhaps the hardness rating of the material is also a factor, softer being “springier” (but perhaps also more delicate).
Your procedure also appears to have resulted in a larger air volume under the sheet than was there originally, you seem to include the 4 felt washer backed rivets in the air space. Any idea what the function is of those 4 rivets? Adds mass to the backing plate which might affect resonance, but I can’t see any other purpose.