We're visiting the US from Europe. Everything in the states of Ohio, West Virginia, Virginia, Tennessee, Kentucky, Mississippi, Louisiana, Texas, New Mexico, Arizona, Utah, Colorado, Kansas, Missouri, Illinois or Michigan will do...
You could try NutHouse Industries out of Ohio. They were at Expo East and West last year. I'm talking to them about building a big custom box at the moment.
Most decent welding shops can fabricate one for you. Just remember that aluminum is roughly one-third the weight of steel - and three times as expensive...
I had these guys build me 2 completely custom aluminum dry boxes for my trailer. The quality of the boxes I had built was outstanding (double welded) and I really like the "tumble home" style with slanted fronts.
Here's a shot of one of the boxes on my trailer's tongue rack.