I'm thinking if it's unpainted (I think it's cheaper if I have it zinc-plated, rather than powder coated), could be under $100 shipped and if it were stack-able, that'd be pretty cool, that would make it modular.
Perhaps something that houses 8 gpu's, a motherboard and a PSU. And something that's designed to have 120mm fans both front and rear.
Maybe the motherboard tray screws in and is removable to have 16 gpu and no motherboard rather than 8 and a motherboard.