Oh I forgot to say I reused the PCI slots from the C= case and pop-riveted it onto my new back, but as it only had rivet holes down one side I welded the other side to the back plate in 3 places. It realy helped with stiffening the whole lot up.
I opted for .6mm mild steel over aluminium in the end and painted it in black stone chip, basically as it dries satin, not too glossy, and i had a can handy.