Update: Detailed video tutorial! youtu.be/raBfRj1SW2o
This is a companion Thing to my Micro-Adjustable Bare Wire Z End Stop ( thingiverse.com/thing:23811 ) -- this is actually the way all of my end stops have looked since January.
This is about as inexpensive as you can get with your endstops, and yet they've worked flawlessly and predictably for months on end. Each endstop is fashioned from a paperclip, a short piece of copper wire, some electrical tape, and a twisted pair from an old network cable.
They are NO (normally open) switches as opposed to NC (normally closed) so you'll need to configure your firmware accordingly, and you don't get the fail-safe you would otherwise get with NC switches -- but the trade off here is in the simplicity of the mechanical action, arguably lower risk of mechanical failure, and the lower vitamin count.