Small update.

Sticking a bolt in from the underside wasn't able to be done. There is a chassis rail (or something like it) on the under side of the car in line with where the sub box sits. But, there was a plastic plug in that area (when looking in the boot) and when I pulled the plug out, there was a hole about 8 to 10 mm in diameter.

I screwed a long screw into the bottom of the sub box, and when fitting the sub box into the corner of the boot, the screw that is sticking out on the underside of the sub now slots into that 8mm hole already there, and the theory is it should stop the sub box from sliding about.

I'll update this thread in a week or two.......