I looked at machining them. There isn't any room in the bellhousing to increase the cross section where the critical stresses are found, and it would need to be machined in at least 3 or 4 different setups. The worst part is since most steels have roughly the same youngs modulus, you can easily stop them from eventually breaking- use 4140 or 4340- but to make them stiffer so you get all the throw out of the PP, isn't very easy. All you can really do is harden up the steel somewhat which is why Q was quenching.
I think a machined part could be made to be stiffer then stock, especially if the transmission case was ground down to give a bit more room for install, but when i looked at it, it wasn't cost effective.
You certainly don't want to make one out of aluminum regardless of the tensile strength of whatever alloy.
FWIW, these do flex significantly in regular usage even when they aren't "bent"... Like a bit over .060" with a single diaphragm "race" style pressure plate. It may be what causes a lot of these transmissions to drag clutch disks at high rpm. You can't shift a transmission that still has load on it.
PS: Found oooooollld pictures.
I don't think I ever got past "seeing what i could fit in the bellhousing" so that certainly isn't a production ready design by any means. I already see some corners that would need a radius or they would be impossible.
Modified by pete@integrated at 2:52 AM 12-15-2009