David,
You may need to get a standard Beseler 4x4in. drilled. Do not remember if you said, do you have a 23C or 45? This will make a difference as well, have you checked with Camera&Darkroom there off the plaza? Would think they might be of some help, at least they should know if someone local can drill a blank board for you. Also, have you located a 25mm lock nut or is it for a threaded lensboard? (sorry not familiar with the lens).