Both the 110mm Toyo boards for the field cameras (and, the CX) and the 159mm Toyo boards for the monorails and 8x10s are made of metal with the edges moulded over to 90°. The inward-facing edges fit into a little trough in the standard, forming a light "seal". Thus, if you make your own boards out of wood or other material, you'll want a "stepped" edge to form a light seal with the inner edge of the opening in the standard. Plus, you'll want a thiner area around the lens hole for the retaining ring. A difference of about 3/8" in the hole sizes usually works well, but measure the retaining ring of the specific lens to be certain.
With thin materials like Pergo or other flooring, the easy way to accomplish both is to simply laminate two pieces, having cut and drilled the inner piece prior to assembly. Be sure to run the grain of the inner board at right angles to the grain of the outer board for greater strength and warpage prevention. The corners of the Toyo boards are also rounded slightly.