Well, one thing you could do (and this by no means makes it right or best) is use a roller switch and a cam plate. Here's a pic of a roller switch and a link to a place where you can order it.

http://www.radioshack.com/product.as...5Fid=275%2D017

With this set up, you would need to fabricate a cam plate or something similar so that when the switch gets to the point that you want it to open, the roller would drop down off the raised cam and open.

I'm sure there are probably better options, but if nothing else, this one would work.

Good luck,
Firechicken