Are there any super-strong cases out there that are still fairly slim? I would expect one of the cases would have to be made (at least partially) of some sort of rigid metal on the case back - i.e. carbon fiber might not be a good route? Would a metal rear band be possible, especially if the design incorporated the bumper edges (increasing strength), without adding substantial bulk/size? A standalone strong outer bumper (Element Case?) could also be pretty effective when it came to rigidity no?