If you've got access to a bucket of Lego Technic pieces, then you've probably already got everything you need to build what could possibly be the most useful keychains ever devised. As for the building instructions, just take a look at the image above and you'll easily figure it out.
It's cheap, it's compact, and removing a single key doesn't require you to tear your fingernails to shreds trying to pry open a keyring. And because Lego is produced to such exacting dimensions, there's little to no chance this thing is going to come apart until you want it to. As a bonus it's even easier to build a spot to hang the keychain when you get home—just stick a Lego baseplate to the wall near your front door. Done.