Busch C are really fairly simple
I have made many boards for the Busch C out of different materials - Alum., model aircraft plywood, hardboard, even thick paper in a pinch. They are really fairly simple. Cut 6 cm x 6 cm square, locate center, drill hole to fit lens or flange, drill corner holes, sand round the corners. I have only wood shop tools. But, because you can make the board to your own specs., I would leave it square, nix the corner holes, and use a sliding lock to mount.
On the C I use the most I modified it with slide locks for easy lens changes, it also has a tilting back that has sliding locks for 23 roll flim back, also uncovered wood, a very pretty little camera.