I started with this article by johnnyoptic on diyphotography.net. As suggested, I bought some lenses from www.surplusshed.com. I also picked up various sizes of PVC connectors from Home Depot. Finally, I ordered an M42-to-EF adapter from eBay (because M42 was the cheapest EF adapter I could find), and some JB Weld epoxy.
I considered cutting up a spare body cap instead of using the M42 adapter, but the adapter has the advantage of locking into place like a real lens. Also, the 42mm diameter of the M42 threads is conveniently close to 1 1/2 inches, the diameter of the PVC.
Eventually I arrived a combination of lenses and PVC that seemed like it would work. I confirmed exact positioning by wedging the lenses in place using tissue paper. Once I was sure of where everything should go, I blacked the insides of the PVC with a sharpie (too shiny, as it turns out) and epoxied everything in place.
Once assembled, I took some test shots and figured out the focal length and aperture. I was hoping for an oddball focal length -- 30mm is a bit disappointing, as I already had a 30mm prime. I could have checked this before any epoxying, but I didn't think to.
There is some slight vignetting, and I'm pretty sure it's caused by the extra PVC at the front of the lens. I could cut that off, but I don't care enough to bother. Also, I think I'd see better contrast in the photos if painted the inside with a flat black paint.
I'm looking for suggestions on how to decorate the outside. Maybe a red "L" ring for starters?