I've been looking at this too. Sears sells a black diamond plate box for the back of a pick up truck. They have two sizes. The smaller size looks like it would fit very nicely behind the seat. If I recall, is is about 8" by 16" by 16", has a waterproof seal, a locking clasp and it is expensive. But quality usually is expensive.
The hard part would be building a bracket to hold it in place. I think it could be supported by building a bracket that attaches to the ROPS base on either side. It would hold the box up high enough to clear the top link. You'd have to reposition the Slow Moving Vehicle sign, probably by using an adhesive one right onto the box. And in the case of anyone with a rear Auxilury Light, that would have to be moved as well because, at least on mine, it is in the way. I would think a simple offset bracket could be made to move the light over a bit without getting in the way of the rear turn/hazard lights.
And the bad thing is it would probably prevent you from having access to the factory tool box. I didn't take formal measurements, but it might be possible to have LIMITED access to the factory box from under the new box.