Tony, aluminum may or may not suffice depending on the surface area of the box. Aluminum does have the ability to get a big harmonic in it and flex, you'd be using a minimum of a .120" sheet. Adding external ribs would assist.
However you've got a mill. Machine a billet cover for the thing with external ribs and it won't flex
Alternatively, you could make an aluminum box with a quasi-isotropic top plate of carbon fiber(VERY ridgid) that you bolt down with a seal
so many options
