A lot of people i guess have fitted subwoofers and everyone thinks their solution is the best
Seriously though, I tried first to fit an 8 inch woofer directly in the standard 6,5inch mount by using an adaptor plate. It looked nice, but it was hard to get the damn thing airtight between the plate and the metal sheet. The metal behind the carpet is not flat at all as one might think. Also it is not rigid enough.
So instead i made a small sealed box of 10lit volume and dropped it in the storage compartment, I also bolted the amplifier on the longside of the box. The amplifier had to be bolted on the box when the box was in the compartment, otherwise it would not have been possible to squeeze the whole thing in through the opening. I also added some noise killer mats on the inner walls of the compartment. It sounds much better now. The box i placed as far to the side as possible so there is a liiitle space left between the box and the battery, for the femal codrivers handbag and some other small bits.
I am actually using a DLS woofer and the box size is not optimal. My plan is to go for a RockfordFosgate 8inch woofer soon, that will need an even smaller box and will give more bass.
But the best improvement was to make adaptor plates and mount a decent 5,25inch kit in the doors.