Now you have me thinking FLexpat, regarding the home-made vac. I'll do some research.
I've read that about the shims, or wedges around the perimeter. I was thinking door frame shims, but it'd be nice to have it continuously supported all the way around.
There's just a bit of stubborn plywood remnants left; I could probably grind at it at this stage. Tonight I'll tape off the holes, lay it flat, and fill with water to soak. It won't be much weight, but it also couldn't hurt in flattening it out some. Maybe support it on saw horses along the long sides only to see if it helps with the low-spot in the middle. Maybe stick a few pounds in the middle also.
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