I bought the box years ago at Thoresby Hall Sunday Market with crappy 10 inch subs and it stood in my garage for years until I got the smart, but the box was too deep so the bootlid would'nt shut, so I measured the space and found that the Sony Explod 10 inch subs would fit with the cut down box, the box is ported towards the passenger compartment so it lets the air out so you get some nice bass, as there is not a lot of room in the boot to get the best sound.
I never need the space in the boot anyway, the same as you, if I need to carry stuff, I take the big car.