Long time ago I had an idea of a mirror, made of thin foil covering a metal plate with shallow cavity (Mylar was not available then), and the parabolic surface would have been formed by lower pressure of air between the metal plate and mylar foil. I even contemplated to patent the idea :-) I even made the small prototype and it even worked somehow
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Another method of parabolizing of such mirror could be achieved by applying the high voltage beteeen the two ..
In both cases such mirror would have been adjustable in terms of focal length..
But the mere fact that the mirrors are today made of pyrex for so long now it telling us something: that glass is THE way to go.
Other methods are just to fussy and /or expensive or not good or stable enough.
B