My car has the kinderseat so I lose undershelf storage behind the seats. There's a little box under the sideways seat but not much.
It's also bugged me that I've been carrying around an old bag full of tools and spares since I had the car. Sockets, fuses, screwdrivers, tape, bulbs, belts, pliers, etc. I got to thinking about that space under the softop well right at the front of the trunk.
A bit of measuring, a hunt around on Amazon and I think I found a good solution. Two plastic boxes. They came clear, so I sprayed up one side with some matt black Hammerite. Stuck some velcro (and felt) underneath them and plonked them in place. They won't move, won't rattle, hold a bunch of stuff, and are much tidier than my old solution. I labelled them as my wife is soon to take the car across France again. Hopefully she'll never even know the boxes are there but just in case...