The brake light switch I'm using is a small lever-type microswitch that I had in my junk box. I've seen these or similar ones available in Radio Shack. I made a little bracket out of aluminum and drilled a couple of holes in it, then carefully bent the switch lever 90°.

http://img180.exs.cx/img180/3755/f0000003dce5wo.jpg

Here you can see the bracket and switch mounted on the bottom side of the rear brake lever.

http://img180.exs.cx/img180/8201/f0000006dce9aa.jpg

I'll keep an eye out for a waterproof microswitch.