I'm not a sound engineer but here's an idea. I've seen 'castles' for re enactment built this way.

You'll probably need a flatbed trailer to haul it on. Get as many sheets of 3/4" plywood (4'x8') as you need to make the room large enough. At least 2 'slide bolts' for each join. Cover one side of the sheets with cork--they still sell the 12" cork tiles. 2x4 pieces cut on the angle for bracing the corners. Toe screw one end into a corner piece, or drill holes in each corner piece and lash the bracing in.

Cover with a large tarp if desired. You might also haunt 2nd hand/thrift/charity shops for heavy bedding to hang over the cork, or use instead of the cork. Rugs for flooring optional.