Decent LED taillights are so inexpensive and so much better than jury-rigging your reflector that it's not worth the time or cost to modify it.
They are self contained, run on AAA batteries that last a long time and have both steady and slashing modes. This one is only $7 and is better than anything you could make:
http://www.nashbar.com/bikes/Product...42_-1___202551 For a bit more money you can get a super bright taillight that can be seen for blocks.