Summary

This switch closes a circuit when there is a magnetic field nearby. It may not be as sensitive as the ones you can buy from electronics manufacturers, but it works.
https://www.youtube.com/watch?v=b4piw_LMiRg

Instructions

You will need two 4mm-wide wall hooks (the kind used to mount pictures). Make sure they conduct and are attracted to magnets. You will also need a hot glue gun.
Print two of the attached STL files (one of the mountable variety if you plan to mount it). Flatten the wall hooks with a hammer to get them completely flat, and lay the first one on on of your printed pieces leaving about a half an inch to spare from one end, and hanging out the other. Place a dab of hot glue on the end where it is hanging off. Do the same with the other half, and then place the halves together so the metal pieces are not quite touching in the middle. Seal the metal in place with hot glue.
The hot glue lets the metal move slightly, so that one metal piece bends to touch the other when a magnet is near either side of the switch, completing the circuit between the two metal ends still sticking out.

Reed Switch