N8N wrote:
....

shim stock or shingles could be made to work but would require a lot
of planing/sanding, is there any wood product that you can think of
that is flat and 3/16" or more thick (but obviously not so thick that
it would require excessive planing/sanding when done) ...
....

A small piece of pine w/ decent grain and a (sharp) block plane will
make quick work of it.

--