I am definitely NOT a carpenter. But with some scrap pine, sheet rock screws and a circular saw, I built this. It takes up all the space behind the back seat, and is secure with the top off: you need to unlock the back door, then reach your hand in the hole, then unlatch it. I would tell the shape, but I am not that good with geometry- some sort of irregular trapezoid?
It has my sub-woofer in it, and space to lock up a few things- tow ropes, hand winch, first aid kit, etc.
Not the prettiest- but it works: