The speaker box is held in by a steel bracket under the tray, none of the tray was cut to fit it, the two subs are mounted on an angle, one port to the rear of the subs and one to the front of the subs, The seat fits back in it's original place and is loud enough to make your vision blurry. The whole system is water tight.

Why not build a fiberglass molded box ported to the right volum accroding to your subwoofers's specs? The fiberglass box takes time but looks great when done.

I've seen 2 18" subs that were mounted in an integrated fibreglass box behind the rear cabin wall.
Had strategically cut holes into the cab wall.

Not my cup of tea, but it was neat, tidy & shook the whole car.

The box that the subs are mounted in are exactly matched for the size of the speakers, and even a 10" woofer is getting near the upper limits of what you need for a 1200 cab. This way it doesn't interfere with the deck, so the bike can still fit on the back.

i have finally come up with a solution for the sub in a 1200 ute, this solution involves no cutting of the tray or wall. All up it cost me 350 bucks not including head deck and im very impressed with the performance. It is like a bass cannon tube. it has a 6inch sub which is ported through this cannon and yes it fits behind the passengers seat in a dato. You have 2 have the seat 4ward slightly but its worth it. This cost me 250 bucks and then a small sony amp cost me the other hundred. A 6inch doesnt sound like much but it cranks along awsum. Its just right not an over kill. I havent got the name on hand but can find it if anybody is interested. I got it from a shop called JB hifi. I recommend this if your keen 4 a bit more oomph but dont wanna cut anythin and dont wanna spend 2 much.

Or stick 'em in the tray under a fibreglass lid :D