I found a solution that looks good, was cheap, easy, and adds plenty of storage!
Motorycle tail bag on the passenger side (Wolfman Wolf Tail Bag, available EU/UK and USA). Some smaller storage bags (for emergency road kit, tools, etc...) Lots of velcro to hold hold everything in place.
I especially like the wolfman bag. It's 25 liters of space and big enough to hold a full face helmet. Sturdy, has a few pockets, and zips closed to keep loose items from flying around. I've got the medical and safety kit on top of the battery and the spanner/fix-a-flat/hammer under the steering column.
And still a few spaces for some other small stuff (like on top of the fuse box.
This will be a great help to carry some spares while freeing up space for a long road trip.
