Any comments or suggestions on this project: 8 by 4 foot riser, using 2 by 8's - 16 inch centers -, 1/4 inch plywood on bottom, 3/4 on top, then #15 roofers felt, then another 3/4 inch plywood; then carpet. I'm going to stuff it full of insullation (regular pink stuff). Was wondering about installing one of the Sub woofer "vibration" units in it. Any comments. Is this structure too sturdy to "shake."