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Another question is is the back curved or flat, I'd guess these mirror s are slumped during fusion ? Your support structure will have to accommodate this .
Single points edge support at 90 degrees works well up to a monolithic 20"X 1.5"thick mirror. There are unique astigmatism cancelling properties with a 90 degree support . It is really only below 30 degrees or so that the edge support really mattered too much . Ensure that the edge pressure on both front and back plates is even , but if the whole thing is mechanically square its not much of an issue .
Scott's cell looks very nice and the edge support deals with the face plates .
The thing with cells is that you can start simple and get more sophisticated if you need to .
The finest Mars opposition detail I've ever seen was through a 14" F5 plate glass mirror Newt of 30mm thickness mounted in the scope with a solid ring of 1" Velcro squares at the 60% zone and nothing else . I transported this scope on its side for years and never had any issues with the mirror coming loose. The idea started as a bit of fun but it worked superbly . A wooden spatula was all that was needed to prise the mirror off for coating once .
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