You'd be best getting hold of a Linhof/Wista style board and building the front standard to take them, they have a ring on the rear that helps provide a light trap and so you need to get everything as close a fit as possible.
I'd go for either the Linhof of the Sinar sizes. Sinar is nice as it is bigger but, on the other hand... it is bigger. This may not be an issue, except for portability. If you want to use BIG lenses (shutter sizes 3 and bigger), then may be Sinar would be better (but a #3 can also be used on a Linhof style lens board, if the rear element is not too big.
Long ago there were something like standard sizes. The most common was 4" square and 1/4" thick with a 1/8" deep and 1/4" wide rabbet around the edge. It was used on anniversary model Speed Graphics, many Burke & James cameras, Graphic View cameras, and countless others. These can easily be cut with just a table saw or built up from 1/8" plywood available at hobby shops.