You might want to consider the Stealth "Dormitory" Safe.
It is vertical, not horizontal, intended to fit into a closet, with the access controls higher than would be the case with a shorter safe.
It can be opened by PIN or by one of your credit cards.
It comes with a floor mounting plate to bolt it to the floor, although you have to get the needed hardware locally.
If the battery operating the safe should die, there is external provision to provide power to the unit.
You could, perhaps, mask its presence with a plywood box that would make it look like the box is covering plumbing, etc.
Look it over.
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