The existing ceiling is just sheet iron bolted to hardwood beams. If anything this would amplify sound

and is too high above the room to safely work on.
What I want to do is build a roof to cap off the room that rests on the existing freezer and back walls. How do I go about this?
Everything is being built by a family friend who is a house builder by trade and hasn't had experience with rehearsal studios before. I will be assisting.