Dan, I use a lot of lenses hung out in front of a shutter, sometimes far out in front. I use a simple similar triangles model to tell in advance whether vignetting will be a bad problem. It is highly approximate, sometimes says that setups that work shouldn't, so use it with care.
In your case, my little model comes down to: if [obstruction's diameter/2]/[distance from the lens' diaphragm to the obstruction (in your case the rear of the tube)] > [(format's diagonal/2)/lens' focal length], vignetting at infinity shouldn't be a problem as long as the lens is centered on the film. I think you're home free but since I'm guessing at the dimensions I could be mistaken.
What I don't see is how you can avoid buying a board anyway unless the tube behind the lens is on a plate that fits your camera.
I'm not sure what you'll do for a shutter, either. SKGrimes makes some recommendations ...