Not quite what you're asking for, but I have a little backupz type light,
http://www.cyclestore.co.uk/productDetails.asp?productID=5384
on my helmet. I got a pair last Christmas, I think they came from our local hardware type shop and were even cheaper than £5. The front one is a handy extra as well. Worth having a look and investing in a couple of pairs when you find them cheap - very light and mountable just about anywhere with a bit of thought.
Also, if you are caught out after dark with a mate with no lights, they are easy to lend, not needing brackets...