How about inside the rear taillight access hatches (on a wagon).
Or put it in an envelope and tape it to the bottom of the "junk tray" in the spare tire well. No one is gonna flip it over if it is full of crap, they will remove the tray, sit it down someplace and then put it back without ever looking at the bottom of it.
Or get a "book safe" where it looks like a hard cover book and it is hollowed out inside. Buy a boring ass book and no one will ever steal it. Leave it in the trunk / cargo area. Or make your own.
Stick a magnetic "key safe" to the underside of the package shelf i n a sedan. Only way you would see it is if you were lying in the trunk looking up.
It is still ugly.