[bpape]

Just looking at it - it appears to be kind of a tweener on size. I'll agree you don't want a ton of stuff everywhere - just a bit of bottom end control and potentially something on the ceiling.

To do the corners, no drywall, just cloth. Do as you please on the top. As for filling, if you're doing 18" square, then plain fluffy fiberglass will work just fine and cost you less than 703 or mineral wool. The additional thickness helps a lot.

Bryan