No way, I am doing nearly this exact same project...!
I decided on an 8 inch (203mm) focal length, and just made a wooden box to fit some Lisco 8x10 holders that I picked up for cheap on eBay. Of course, film holders aren't 8" by 10", so I just traced an outline on wood and cut it to actual size. I ordered my pinhole from Lenox Laser in MD. The camera is nearly done, but I want to add some elastic to add some pressure to the back of the camera and hold the film holder in place. I may add a shutter, though the darkslide will probably make a good shutter as well, given that I'm going to start with paper negatives.
I have not worked with paper negatives before, but I have heard that a yellow filter or a 00 filter will help control contrast, and pre-flashing might also help.
Making a 4x5 or 8x10 is not much of a project, is a 30min job.. :) For a project you need to build a 40mm 6x6 roll film pinhole. But if 8x10 is the choice then a 140mm- 200mm range is nice, try sourcing Xray film,l you could get it from here (http://dlc.com.au/en/x-ray-equipment/x-ray-film.html) but check the Christmas thing.
Don't forget to download Pinhole Designer off web (http://www.pinhole.cz/en/pinholedesigner/), and go to f295.org.