Allan,

Black foamcore would work much better than cardboard and at not much more cost. you can find some plans at http://www.stanford.edu/~cpatton/foamcore.html (I built this and it worked very well, without any light leaks). Lee is correct that you will have to 1) cut a slot for the ridge on the filmholder and 2) secure some black felt or something like it to the mounting face. For the second task, I used black flocking paper from Edmund Scientific that I happened to have lying around. It's really great stuff, and pretty cheap, I think any tinkering photographer should have some of it. As far as the rabbet is concerned, it could help hold the filmholder in place, but is not absolutely necessary. When I built my first foamcore camera, I did not build a rabbet and I experienced no light leaks.

PS. I know you said that you had already worked out the focal length, etc., but I would like to recommend that you go really wide ~50mm. At this focal length, the resolution is incredible and the falloff is still quite acceptable.

-Melchi