I bought a generic 6-LED tail light from the local homegoods emporium. $7.00... and it came with my choice of seat tube or rack mount. Plenty bright, and I didn't need to get all McGuyver trying to rig it to the rack.

It's a real disappointment that a $30 light doesn't include a 50-cent piece of plastic and a pair of bolts to mount it to a bike rack.