A trick that I read somewhere, but never tried, is to remove one of the screws from your front or back license plate and put it through the hole in your key (with the key behind the plate). Then, if you need the spare key, it’s just a matter of coming up with a screwdriver or a coin or something to get the screw out and freeing the key.
I think my license plate has bolts in stead of screws which would make it a bit more difficult, but it’s still better than not having a key.