I use to build aluminum inclosures for movie generators. We used 1/16 aluminum, sprayed the inside with sound proofing, foam insulation with acoustic vinyl barrier and then covered with inch thick felt. Felt works pretty good by itself. Sound baffles added and actuators for opening intake and exhaust vents. You could bolt huge rubber mounts to the floor. We could quiet a 8.3 Cummins Turbo to 50dBs at 100ft while under load. You could barely hear any thing.