I made my own linhof pinhole boards from stiff matt board. Black on one side, white on the other so I could also write details on the white facing outside. Bought a full set of pinholes from pinhole resource website, traced a linhof board on the matt board and a steel ruler and sharp "stanley" knife did the rest. Works very well.
Example below of boards and a shot taken with a Deardorff 8x10 which has a linhof board adapter, but no reason you couldn't make any other size board.
If you are particular about it being a light tight fit you can make a thin packing frame from matt board or foamcore to go behind the pinhole board. I did this initially but tests showed it wasn't necessary.