Accepting the challenge thrown down by Ethan at http://blog.makerbot.com/2011/03/09/pinhole-lens-for-nikon-by-xjcrawler01-and-a-wee-challenge/.
Actually, this is something I've been working on for a little while. I just wasn't going to post it until it was really finished. Printing on the powder printer at http://metrixcreatespace.com resulted in an inner diameter that was a hair too small to fit around the Canon 50mm lens I have.
The important measurement in this flange is the height. Placing this on a lens, your focal length will be 42mm from the bottom of the flange to the imaging surface (42.14mm if you measure to the back of a piece of 35mm camera film).
NOTE: When using the original SketchUp file, I had to scale everything up by 10 because it doesn't handle small measurements (< 0.1mm) at all. Where you see an inner diameter of 480.0mm, in real life this should be 48.0mm. After exporting, scaling down by a factor of 10 will result in proper measurements.