A quick peek at my geek project.

1) Three pieces of PVC pipe fitting, 1 1/2" diameter.
2) Beveled inside on the front edge on the first piece - to accomodate the lip on the MR16 bulb. I used a grinding bit on my dremel tool, and some mineral oil dripped on the surface to prevent it from totally melting the PVC from friction.
3) The back piece (pipe cap) gets two holes drilled into it and mounted where the front reflector used to be.
4) Silicon sealant (goes on white, dries clear) glues the MR16 into place and seals the front piece from water. Replacing the bulb will involve cutting the old one out with a razor and cleaning the sealant from the plastic - it'll take the new sealant longer to dry than to change the bulb.
5) The fully assembled and attached light. The middle segment has a hole for the wire to come out at the bottom, which is fed through after screwing it into the front piece. The whole assembly is pushed into the already mounted pipe-cap... It's tight enough to not need any further attachment, however it would be easy to include a short-screw through to hold in place if this was not the case or one was worried that it might pop out. I put Park Tools waterproof grease on the threads and the edge of the light that meets with the cap so that I'll be able to remove it later and to guarantee the seal.
6) Last shot (out of order) shows the MR16 in the beveled PVC piece, with fresh sealant.

No pictures of the light actually functioning yet, as I've not completed the electronics housing which is the white box hanging under the seat-stays. That'll be hopefully completed in November when I can afford the batteries and charger.

The light itself cost around $15 all said and done, for the PVC piping, 20 watt bulb, and silicon sealant. Nuts, bolts, wiring and reflector mount I already had, and the electronics housing is an old plastic box from a camping first-aid kit. It latches from the bottom (as mounted) which will allow it to swing open in the triangle to access the batteries without removing the box.