Black foamcore would work much better than cardboard and at not much more cost. you can find some plans at (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.