If you need to, a hobby shop has thin plywood which can be glued up into the correct thickness for just about any application. Ralph's suggestion about a rabbet cut is a good one, since light travels in a straight line. If you don't have a shaper or a router for the edge cut (essentially, a long notch cut completely around the edge of the board which would normally be too thick to fit inside the groove for the lens board in the front standard), two thin boards can be glued up to make a stiff board which fits into the recess, has the notch acting as a light trap on the back side and mounts the lens properly.

Be sure to make several extra boards if you are going to the trouble of doing this work. Have the blanks made up so you can drill them and glue them up as needed for a copal 0, 1 or 3 as needed. The front boards should fit well into the recess, the back boards should have enough clearance to allow an easy fit around the inside of the openeing in the back side and room for the shutter and mounting ring.