We had the same concerns some time back, this won't help with laptops and tablets etc but for phones, money, wallets and keys this works fine. The yanks love car/4x4 safes and that's where I got the idea.
It's on a stainless steel lanyard fixed around the drivers seat bracket, the other end of the cable in the safe box is inserted and wraped around a pin then bolted to another pin so no one can get a straight line pull to try and steal the whole box.
Once under the seat you can't see it and it'll take awhile to unbolt the seat and or break into the steel box !
Sorry for the more that average photos guys, the camera in the phone didn't like Bushmans on it and it's etched the glass
Cheers
John