This may be a little off topic in this thread, but i've heard rumors of a hard top that is something like a roller door. the rolling mechanism (although it would take up a fair amount of space) would be just behind the cab, and you run two tracks along the top of the tray. then you simply roll it open or closed, so you don't have to totally remove it or have the cover hanging wide open when you have to cart something big around. (like a dirt bike )

The idea of an aluminium one sounds good. something like 2mm plate and maybe a couple reinforcements...