In the UK B&Q sell some aluminium tool cases, but I'm not sure how big you're after - any idea what sort of size? Most DIY places should sell something, and so should any "cash & carry" type stores that have tools. On a similar note, I'm contemplating building a steel carry case for my guitar FX unit out of angle steel and treadplate (the steel sheeting with little raised diamonds to make it non-slip) - it's the most awkward shape out there, so I'm gonna head down to my dad's steel fab place and weld one up myself. Would there be anyone interested in seeing the finished article? It may not exactly be midibox-related but the basic steps can be applied to any enclosure made out of steel.