Eddie just pointed me to this thread. For the 11x14 I made, I used foam core to make a pyramid structure which I painted the inside black. The back was simply a paper box glued and gaffers taped into place, and the lens was simply gaffers taped onto the small part of the pyramid. I suppose that if you wanted to make multiples and use the same lens you could make a little plate hinged on the side, then put everything into a darkroom/giant changing back, take the lens off and close the hing, then tape it all up so it's light tight. Or just have multiple giant changing bags I guess. The foam core is surprisingly rigid and sturdy once all taped and painted. For the tripod mount I simply glued/taped the quick release to the bottom of the camera (which is very lightweight) and mounted it on the tripod.
And here's the part you are probably wondering about; to determine the focus I simply put the lens on my view camera and measured the bellows draw required for the focus distance I liked. I looked into doing the math, but I wanted to experiment with focus distances and find one I liked. For example, my camera was designed for portraits, so I made the focus so that it was a head and shoulders shot on 11x14, which was a certain distance from the lens. I had the lens on the 8x10 so I just "guestimated" how much bigger the 11x14 frame would be and adjusted as necessary. Thus, I knew that if I wanted that head and shoulders shot I would have to move the lens that distance from the subject to "be in focus." So I simply shifted the tripod/camera until it was that distance from my subject when taking the picture. I used the lens cap for a shutter. For framing I suppose it would have been easy to make a wire frame viewer of sorts, but I just guessed and actually got it in two tries (I was using paper negatives). Since I could take the tripod plate off, I could keep the framing from the previous shot and adjust as necessary after seeing the negative.
If you don't have another camera with bellows long enough to test the lens on you could even just get a sliding poster tube and tape the lens to the front and some vellum on the back, then measure the bellows draw and distance from the lens to the subject. This method would make it quite a bit harder to guess where the bigger frame ended though.
I would be happy to help you more with the project and/or send you some photos of mine, but I am leaving for a trip to China for the next three weeks. I will be back September 20th if you'd like to get in touch then. Good luck! These things are fun!