A recessed lens board is the best solution. For occasional use of odd lenses, I've made lens boards from a couple pieces of cardboard glued together, with a hole cut for the lens. In your case, a 35mm lens from your camera kit would probably do. If the lens has a bayonet mount, the hole in the board needs to be tailored to allow the bayonets to enter and then turn a few degrees to hold the lens in place.

I've printed a lot of 11x14 stuff with my own 23C and a 50mm lens using the standard flat lens board. Of course, some "50mm" lenses are really 48mm or even shorter. The bellows will compress more with use, I find, and a spring clothespin on the rail will keep it from extending during use.