Maybe some waterproofed plywood? If you are going to make it 8x8 feet or larger,
1/2" is going to get a bit heavy.
It might be better to use a strong frame, with 1/4" plywood over it.?.
Or, maybe just make a 'light' wooden frame, with a strong center section (for the mount),
and use boards (1 x 1/2?) with a 8" to 12" space between them.
Weather-proof all of it, before mounting anything.
How securely you mount the mirrors, depends on your environment.
Do you live in an area where the weather will be a problem?
Like does it snow a lot there, will the mirrors each get a 20 pound ice load?
Or, do you live down south where a hurricane can rip off the mirrors??
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