I had this issue in my SN95 and also in my friends 00 v6. On the back of the rear seat, inside the trunk, there are some bolts...find one, and make a "L" bracket fro mthe bolt, to the top of the sub box, and screw the L into the sub box, and put the bolt through a hole in the L. Then ut some washers and a nut on the bolt(stud) and you are set. Wouldnt do it any other way, so holes in the floor
I made a bracket that hooked to the LATCH system, since I don't have kids. In a coupe, a simple peice of Velcro under the box will USUALLY hold it in place.