1/2 would eliminate deformation and your attachment would be the load bearer. with lumber costs what they are i'd test 1/4 as well. or tri-wall cardboard.That is a crown stapler. I'm familiar and we have all manner of them ranging from 1/4" all the way up to 1". That part is easy if I can get the plugs made.
Temporarily, I may have to make some 1/2" thick plywood ends to drop in on top of the metal caps. That will spread the load out to the rim and slow down the deformation some.
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