I tore off my wood paneling in order to insulate the block with rigid foam
panels. The electricians who recessed the metal outlet boxes in the block
mounted them on what looks to be not much more than a thick shim, with
splintered edges, and cut nails.

I guess I'd prefer a little neater, more secure, and workmanlike job, using
3/8" plywood and Tapcons.

But does anyone know the minimum size the the wood mounting plate that I
need to cut? I want it as small as possible since foam will be on top of
it, which I'll have to carve out for the plate.

Thanks.