Custom Speaker box seat?

When i got my truck it didnt have anything in the back seat and I have 2 little kids so I am wanting to build a short bench style box to go across the back and have it so the speakers hit downwards and the kids can safely use there booster seats on top of the box. All will be permantly mounted and have seatbelts accesible. Any ideas?

I am in the same situation as you, my only concern is safety. you could build a small enclosure that will sit on the floor and the kids could rest their feet on it maybe a box with 8's. You could also cut a rectangular area under the rear fold down cushions and this way the box will not be that tall.

I have made a sub enclosure like you stated for a friend using shallow mount subs. He also had kids, so I made the box 8" tall with 3/4" mdf board built over a metal frame that was welded to the frame of truck and he used lap belts from a chevy truck for replacement. This design worked well for the kids as their knees were in a more comfortable position. This design was done for the sake of kids and their safety. You could also make a rear center console that holds the speakers.