I recently built a prototype 5x7 camera out of double weight black matt board based on Joe's design above. I've used it enough now to learn what I like and what I don't. The storage compartment is handier than it might sound until you've tried it... the alternative is juggling envelopes of exposed and unexposed paper inside your changing bag. It's a great idea and makes it easier to change paper without fingerprints or scratches.
I too have often wished I had something to sit on... the portable stool is a great idea. A couple of times I've found myself lingering in areas where there was a bench, just because it provided a place to sit and change paper.
Mine has two minor differences:
1) The paper is held in place by a mat window that is attached on 3 sides. To load the camera, I slide an unexposed sheet in from the top. This is easy to do in the changing bag and makes a nice 1/8" white border ( black when contact printed. ). I worried that sliding the paper in could scratch the emulsion, but that does not seem to be a problem, at least not that I've noticed yet.
2) My wooden base is not actually attached to the camera. The camera sits on top of it on the tripod and is held in place by a couple of big rubber bands. That way the same piece of wood with a t-nut can be used for several different cameras.
I'm contemplating an 8x10 wooden version too. ( But for WPPD my eye is on a popcorn container that will hold a sheet of 11x14... we'll see! )