Put a small wire brad on a hard surface. Lay the key over it at a right angle. Now hit the key, gently, with a hammer.
The key will be bent a little, and that should make it stick in the cylinder quite nicely. Start out gently, bend a little more until it is snug.
Or, if bending things goes against your nature, you could clean it off and float a little solder on both sides of the key, just enough to thicken it up and again, make it bind in the cylinder.
Probably could wedge in a little teflon tape or a bit of plastic or metal scrim to do the same thing, too.