I sturdy foam insulation board works well for several reasons. It's light weight, easy to shape, easy to lay (a hot glue gun does the trick)deadens the sound. Try to get the solid foam instead of the partical foam. Particle foam is messy and a waste of time.

Card board is great for "mountain shaping". Using a mesh patern is most effective.

Stay away from foam core as this tends to warp real bad.