I've carried a rifle in a trombone case for a while. Hard cases for electric guitars provide more options, but you'll want to re-work the interior. I have a double pistol case I made out of a double clarinet case-- looks more like a laptop computer case.
There are several companies selling "road cases" for musical gear also, and many of them do custom work to order. They typically have plywood construction with a Formica-like outer veneer, full aluminum corner edging, full piano hinges and rugged draw-latches, and a firm, foam padded interior, which you can custom fit using a sharp knife. Very tough, and very much not gun-like. Anvil Cases was among the first in the business, but there several companies doing it nowdays. For a cheaper alternative, try AudioPile.