Use 2-inch-by-4-inch studs to build two framed wall sections 4-feet long and 8-feet high. Use internal studs on 16-inch centers. Build two additional 8-feet-high wall sections; these are 4 feet minus the width of the two longer walls, typically 3 feet 5 inches, with nominal 2-by-4-inch studs. Frame one shorter wall conventionally, and frame the other to accept the wood door.
Assemble the wall sections to cover a 4-foot square. Add ceiling joists on 16-inch centers. Add drywall to the outside of the booth only. Attach the wood door to the booth.
Line the walls and ceiling of the interior with mineral insulation. Staple burlap to the studs to cover and secure the insulation. The burlap will be the interior finish on the walls and ceiling, so take appropriate care while stapling.