1. Reflect the backwave with an angled reflector that points the sound down into a pit of acoustic insulation. Make it go through a bunch so the backwave is completely absorbed.

2. If this isn't possible try putting insulation strips across the inside rather than just lining the walls. The more insulation the backwave has to go through the more it will be absorbed. Spacing is important too so it is better to have two or three sheets spaced apart than one thick sheet.