locked drawers under a locked canopy, with bolts and ammo locked in a steel tool box is how i do it, it`s not just about the laws, it`s also about keeping your guns safe from others. not alot of room behind the rear seats to store things.
If you moved the jack, you could probably do it. We fitted a whole pile of stuff behind our DC back seat. it would just be a question of length and how you're going to mont it.
Also, as someone has pointed out legality?
You could consider making dual storage trays with one side as a gun safe. What grade metal does it need?.
I put my rifles behind my seat. its water proof and alarmed unlike my canopy. im looking for a way to lock it. You can fit 2 or 3 rifles on top of each other in their bags.