Not a good place to hide a car key, a bored kid prank is to steal license plates, got my wife's one time. I put fancy security screws in it. My truck I can reach behind the bumper so I put bolt and a pair of nuts locked together so you need a pair of wrenches to get them loose.

WRT fireproof safes, made sure you buy the media safe if your going to store a backup hdd, usb flash, dvd, etc. a plain old fireproof safe will get too hot inside allowing the plasitc to melt. A gun safe (an actuall safe, not a cabinet) is a better option, much more space and better protection than and of the small portable safes, price per square foot it much cheaper.