Thanks! I think I'm going to start with the mylar/mount pieces for now, so I can get these mics running. At the degree of stretch required to have the same stiffness of .004" glass, I think the tension will be pretty even. The tighter you stretch it, the more even it tightens. Also, if you heat it up, you can even out the tension before you stretch it out, then it'll be pretty good once it's glued on. You can also adjust the damping of the mics after assembly - the clearance can be changed after the fact.
The second phase is going to try one of the machining methods. I think PRR's suggestions would be good to try. I can certainly do some accurate machining on my lathe, so... I also have a sandblaster, so perhaps another approach would be to mask a 1/2" part of the glass, and just blast away (slowly) what's left.
The source glass is a #0 microscope cover slip. I tried to find a #0 cover slip in 1/2" round, but could not do it. I can get #1 in 1/2" round, but that's about 0.008" thick. I'd have better luck with Mylar rather than trying to thin out a #1 slip down to a #0.
-Dale