Doesn't look to tough, a sheet of wire rack, and angle iron. Tough part would be putting it together a uniform and compromising way that both uses the most space for storage, but also will allow a large item to be loaded. I'm working on a storage solution, but it rolls out of the doors, so that an item can be placed on the roller floor, then the roller can be closed, and that item, when the cart is in the closed position is right behind the 2nd row seats. Also figured I needed to make it romovable just in case I need to reinstall the third row seats for some reason. I do like the idea, but I don't know about building it above the bottom of the windows, you know, line of sight type of thing. Definately come in handy, but that means you have to crawl into the back of the truck to put something in the cage! Just my thoughts, hell ya go for it.
The majority of the material would be an angle iron frame, 1 1/2" x 1 1/2" maybe 1/8" thickness. Start there and by a stick, maybe $25 or $30. Make up a few of the cages and see how they would fit together and mount into the truck. Give it a shot, personally I like the idea as long as it doesn't interfere with vision!