Here's my latest build - an aluminum 4x5 pinhole camera for use with standard 4x5 cut film holders. It's a simple and reliable design, and very lightweight. It's made from a single piece of aluminum and joined together with my signature hockey tape construction. The shutter is the standard cable release-activated guillotene style. The camera back has a foam rim under a layer of cloth tape for a light-tight seal against the film back. The film holders are held snugly in place with large rubber bands cut from a motorcycle innertube. It's very low tech, but the camera functions very well. I've also mounted a standard tripod mount on the bottom side in the same way I did for my homemade folder.
The camera has a focal length of 60mm, which is very wide for 4x5 film - about 115 degrees of view (the 35mm equivalent would be an 18 mm fisheye lens). The pinhole is 0.30 mm (f200). I've tested the camera for light tightness with a 6-hour test in hazy sun with a darkslide pulled, and no light leaks were detected.
Here's a test image from the camera - this is the microscope I use when making pinholes. This is a 6 min exposure onto Ilford Multicontrast paper. The abililty to shoot with paper in a 4x5 folder is a nice option - it's cheaper than film, and you can watch the negs develop under a safelight. Then it's a simple matter to scan and invert using Photoshop.
Here's a large blowup of the above image to demonstrate how SHARP this pinhole is. You can see incredible detail and resolution. 0.30 mm is pretty much optimal for 60mm, and I took my time with this pinhole. Seems to have paid off!
This one's on the auction block too. I'll be building another for myself soon anyway. Click on the Ebay link on the sidebar to see my current auctions.