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This is the final product: A surprisingly strong magnetic key hook that takes just a few minutes to do. It is ridiculously inexpensive and does not involve replacing your existing plastic switch plate. I did not even use tape or glue. This is so easy I debated not even posting it. However, someone else may find the information useful. Keep in mind that you need to have the cheap, ubiquitous, builder's quality, plastic switch plates to make this work. The magnetic force will easily cross a plastic switch plate. Any other switch plate is YMMV.
When you are done, you can turn the light out and take your keys. I already have 2 key hooks mounted on the wall next to the light switch. However, my wife has insisted on using both hooks to wrap the 2 rings she has on her key chain over top my keys. I constantly need to take her keys off the hooks to get to my keys. Instead of make a Federal issue over it, I made the switch plate into a magnetic hook for my car key.
Step 1: Begin with a strong magnet and something to wrap the sides
Begin with a strong though small neodymium magnet. You can find them at hardware stores and on eBay. I had one lying around the house. I just found a rubber cap I had around the house. You don't have to use a cap. I needed it to bulk-up the magnet to wedge it between the switch plate and box. You could just as easily tape the magnet to the inside of the plate or glue it.
Step 2: Put magnet in cap and put it behind plate
Remove the plastic switch plate cover and put the magnet in the cap and wedge it between the switch and electrical box. You could probably wrap some masking tape around the magnet if you needed to bulk it up to wedge it or reduce the attraction to the adjacent switch.
I did not cut the power to the switch but I advise others to do so. Realize where the wires are attached to the switch. There was one screw on that side of the switch for the hot wire. Therefore, I put the magnet next to the green ground screw at the bottom. That's it! You are done! The result is a surprisingly strong magnetic hook for keys (as seen in the first image).
Now I don't have to constantly take my wife's keys off the key hook to get mine. Yea.