I've noticed newer autos with airbags have many wires under the seats (which detect whether a person is riding in the seat, for instance, for deployment purposes). As a result, my 2000 VW and now my 2004 Toyota both have "guards" -- if you will -- which make it nearly impossible to stick your hand or anything else under the seat. God help you if you drop your keys, money, anything of value because it requires a significant operation to retrieve these items. As far as sliding a 4" tall metal case under any of my seats, that's a laughable proposition. I would be required to put it in the trunk, which sorta defeats the purpose of having a light duty aluminum safe for me, anyway. If the thief can get into the trunk that safe isn't going to stop him. And, if it's just attached with a cable, a pair of handheld cutters will cut any small cable quickly.
If you're set on a small safe, get a $20-30 Sentry fire resistant safe of about the same size. It's probably tougher to force open and it offers protection from heat, minor protection from fire (like 30 minutes) and maybe even flooding.
Alternately, I think that the glove box is an okay place. Have a spare glove box key on the keychain with a detachable keyring so you can access it without removing your key from the ignition. It's lockable, light duty, and doesn't take up any space. Hard to break with no tools but probably easy to break with a prybar. Just conceal the gun in a McDonalds brown bag to appear like trash. Outta sight, outta mind. Probably good enough for me.