I working on a show that calls for a LOT of thunder and lightning on stage. We currently have a "Thunder board" (a piece of metal attached to the wall with a rope to move it) that is made of 1/8" galvanized metal that is 3' X 9'. However, the director does not think it is big enough or the correct material to create a long rumbling thunder. We are trying to create thunder that is LOUD, it's a box set, and continues to rumble for several seconds afterwards. Does anybody have any suggestions on accomplishing this? We're really trying to avoid recorded efx as we would rather have something live. What size is normally a good size for a thunder board? Is there any particular material that works better than any other? Your help, thoughts, and suggestions would be most appreciated.