That box looks cool when the seat is up, but a little wacky when the seat is down.
Fiberglassing isn't hard, just time consuming, and smelly. If you don't want to be itchy for a week, don't do it =)

You can find MDF at pretty much any building store, e.g. Home Hardware, Home Depot, Rona, etc....

I'm in the process of custom-fabricating a box that will take the place of my back seat.

I kind of like that flip down feature. You still lose a little storage space though. It might be cool to have two separate boxes at either end that swivel so you can keep them pointed towards the hatch.