Wood Plate for Mounting Elec Boxes on Masonry

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.
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Have you considered fastening them up with construction adhesive?
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I pretty much always do that under Tapcons in block.
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There isn't any minimum size wood plate. Depending upon how much you're going to fur the wall out, replace the existing boxes with suitable depth boxes and reducing cover plates to accommodate that depth, and screw them to the surface of the block

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