Step 1: Stuff You Need
- Some Polycaprolactone granules. Also known by the trade-names "Shapelock" and "Polymorph", it's the polymer that becomes soft at 60 degrees C.
- Some elastic cord.
- Two neodymium magnets, bought from an on-line specialist in high-power magnets. One is a .75" disk. The other a .5" disk with a countersunk hole. Though I'm fairly sure a plain, smaller magnet would've worked just as well.
- A thick split-ring salvaged from an key-ring.
Step 2: Cut Plastic From Key.
Step 3: Making the Key
By using a neodymium magnet with a countersink hole, a conical plug of shapelock in the hole is sure to hold it firmly in place. If you can't find one, try a more-common disk magnet, making sure it's completely covered in the glob of plastic . After all, you can always warm it up and disassemble it to try again.
Important! Neodymium magnets may loose their magnetism at 80 degrees Celsius. Polycaprolactone only softens at 60 degrees Celsius so you've got to keep the polymer within the 20 degree temperature window when making this. I just put some granules of the plastic in a metal dish and carefully used a hot-air paint stripper to give them the absolute minimum of heat neaded to soften the plastic. Luckily, the stuff goes from white to clear the moment it's ready to use.
Step 4: Attach with Elastic Cord
Tie the key to the bag buckle with the elastic cord. Use a bit of trial and error to get the right cord length. You want it long enough for you to hold the key at arm's length without needing too much force to stretch the cord. You want it short enough for the slack loop of cord to not get in the way when you're riding.