Is there a reason why you want the enclosure to sit on your backseat? It is not really a good idea over all.
The better option would be to fully remove the seat and build a "floor" out of 2x4's nice and level attaching them to the floor of the car. That also gives you a way of mounting the enclosure to ensure you won't be in harms way should the need arise to hit the breaks quickly or strike something bringing you to a sudden stop.
Well i just figured that it would be better standing up. i never thought about it smashing me in the back of the head lol. well i had them in the trunk and it really didnt feel good till i let the seat down. i just got tired of putting the seat up and down. and a wall was the first thing that came to mind. till yesterday i went to a place to get a price quote they said 2000 to 2500. so i now think i will just get 2 12 ported boxes and stack'em. i dont even really drive my car that much.
If you don't want the back seat anyway......Do a FORWARD facing two 12" enclosure in the seat opening if possible.... Inset it back several inches and SEAL it off from the rear.........Bam!!!......Problem solved.....you can have a LOUD car and No loose box to fly around and kill someone........