Cateye makes a rack mount which fits Tubus rack hole spacings perfectly (50mm/80mm).

http://www.amazon.com/CATEYE-5445620...dp/B009YM2I28/

Cateye also makes 9 tail lights to fit this mount, naturally. This one looks like it would be relatively unobtrusive on a rear rack.

http://www.amazon.com/Cat-Eye-Rapid-...dp/B00BFG832U/

There may be brighter taillights available that can be fit to the Cateye rack mount. It sometimes seems like good lights are sold combined with bad mounts (and vice versa). I ended up making my own mount to fit a Dinotte 160 tail light to saddle rails with pvc pipe/adhesive/o-ring/paint.

If you regularly use lights which require AAA or AA cells, I suggest getting some Eneloop cells and a charger, which are a lot cheaper than disposable cells in the long run.

Also, don't rely on a single tail light. I always supplement the bike-mounted light with a helmet-mounted light (attached by zip-ties).