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Muffin trays have ten or a dozen "truncated conical shape" bits that might be worth experimenting with but I fear the available movement is inadequate for anything useful.

I've just made 2 nice Diaphragms for my 2-cylinder Gamma-project.
The idea comes from this video: [youtube]https://www.youtube.com/watch?v=ctH0noEYh2A[/youtube]

It is basically Rubber pond-liner that is pressed, and heated, so it stays in the new shape.

Here are photo's how i made it. The first 3 pieces were bad, the fourth was good, but not the right size. Then i made 2 of the right size:






Maybe this helps some people, i think it is an effective way of making a diaphragm, you just need to practice a bit, but i believe most people are able to do it.

The silicon rolling sock seal is the path I'd like to take. The simplest solution in to long run would be just to buy them, as Alibaba lists a dozen manufacturers that will build to order. I've never ordered from Alibaba and my guess is that the diaphragms are probably designed for truck suspensions or brakes, so massively overkill, so I'm going to try to just make my own out of standard RTV silicone using an improvised mold (cups coated with mold release possibly filled/wrapped with concrete for support. My guess is that the diaphragm won't be strong enough so the next step will be to repeat with cloth for added support. I'm trying to figure out now which side the cloth should go on, inside or out. I think inside as that would add abrasion resistance, but it also might lead to delamination. McMaster also has a fiberglass reinforced silicone sheet which would seem to be a natural as the silicone would bond well to the silicone.