hey dzk,
i ran into the same prob with my civic, heres what i did.
instead of putting a box in that trunk...i turn the trunk into the box.
1. get you some really good plywood sheet board
2. use the thin partical board thats in the trunk as a die cut out for the plywood
3. cut your shape and then cut that plywood piece in half(easier to get the two pieces in the trunk that way)
4. remove spare tire and anything else that might be below ie jack etc.
5. place the boards in so you can put the subs upside down over the area were the spair tire well is and make sure to leave at least 6 inches inbetween the subs for your ports. mark or score those spots.
6. cut out the holes for the subs and ports
7. i used cardboard tubing that was at least 1/8 inch thick for my port tubes, 6 inches long and use a very good glue to glue the ports in.
8 take the carpet the was in the trunk and cut in half just like the plywood, if done right you should have at least 1 inch to 1 and a half inches left over on all sides
9. once again using a very good glue glue the carpet to the plywood, wrapping the extra carpet over the edges and let dry overnight.
10. cut carpet from the sub and port holes
11. install subs in each piece and one at a time put them in the trunk.
12. make your own grill cover or if you have ones already put those over your subs and bam you got your custom install of your subs and still have trunk space.
hope this helps, the only thing step wise i left out was putting dynomat down in the bottom of the trunk for better sound
peace
Lance