I use a rear light made by planet bike. It's not very big, but it's an insane strobey discoey thing that really gets attention. It' called the Super Flash, was fifteen bucks, and well worth it. It's got a thing like a belt clip on the back that fits perfectly into one of those screw in back things that came with my Topeak Explorer. Check it out.
www.planetbike.com. I've been riding with it for a couple weeks now, and it seems secure enough, but is easily removable to use on my other bike.