We have a PVC conversion and I fitted our safe under the rear passenger seats, I drilled and tapped the seat belt framework and bolted and sikaflexed the safe in it is now virtually impossible to remove it.
I never get the hiding a safe issue, if you find a good place don’t bother with the safe
hide your valuables there.
If you buy a good safe then find a location you can secure to and I mean seriously secure at the end of the day the scums will find a safe but it they can’t get it out easily and cannot break into it your valuables will be secure and you can actually access the safe without a load of hassle.
I got one of the Pacsafe bags I mentioned above. The whole thing has mesh all around of 2.5mm stainless steel cable and the top locks shut and around a fixed object with a padlock (though apparently the padlock that comes with it is prone to becoming stuck locked). I certainly wouldn't put it the same category as a safe, but it is a good discrete anti snatch and grab device.