but for 12's...the only ways i can see fitting them under the seat is by fiberglassing a box. it would not fit flush w/ the front edge of the seat though. or build a bracket that will lift the back seat ~3".
(i almost built bracket to lift my backseat in my silverado...but when you look at how the mid 90's silverado's back seats are mounted, you'll see that a bracket wont be easy to build.)