I've been successful using this one on my s2 riding "roads" where no road bike should go:
http://www.excelsports.com/...major=3&minor=24 It has two sets of grooves running along the long dimension of the mounting surface that keep it centered on the tail of the foil (look at the last pic - I'm pretty sure that's a Cervelo).
It's too bright to look at on flash mode, USB rechargeable and cheap. Check, check, check. I think exel has free shipping for >$99 right now, so you'll have to find $69 more things to buy to qualify for that.
I suggest good, warm shoe covers, provided you've already got a headlight:
http://www.excelsports.com/...;major=5&minor=5