Will, we've done stainless chassis at Marksman in St. Michael, MN
Of course you'll need a good drawing, preferably in some sort of computer file, like a DXF. But they can make parts from an actual drawing- don't send it on a napkin :) my advise is to get only a very small run done at first; if there are bugs in the chassis or drawing you won't have paid for a big pile of them.
If you really want to be classy you can have the corners welded and ground- then have the whole thing polished.
One thing to keep in mind- you might want to do this in steel with a chrome finish because stainless provides almost no shielding. We get away with using stainless because our amps are fully balanced and differential, which prevents a lot of noise pickup so we really don't need the shielding.