I carry quite a few tools and I finally got tired of them rolling around back there. Even in a tool bag the carpet would bunch up, or it'd slide and topple after some spirited cornering and I'd find them in a different place every time I opened the trunk. I'm kind of a neat freak with my car too so I wanted them stowed nicely. Here's my solution.
My tools and a bag just long enough to fit my torque wrench:
I bought a strap for a tool belt and ran it behind the trunk moldings so that it would secure the tool bag into the corner:
The plastic moldings were too flimsy to really cinch the bag down, so I made a bracket using some spare metal, metal snips and a drill. I cut it just right so that it is secured by one of the screws holding in the tail light assembly.
On the other side of the opening for the jack there is a convenient triangular hole just above the wheel well
Using a razor knife I cut two slits in the plastic moldings for the belt to fit through.
Put the bag in place on its end and snug it down!
Or like this:
Nice tidy trunk!