Hey Smokealotamus
Get some cardboard and a hot glue gun.Cut the cb to fit ,put a line of glue across one edge of the cb , take the roll of mylar and roll out about 2" and press it in the glue line.The hot glue tacks in mins. roll out the mylar keeping it tie about 16"and put down another line of glue.Roll on and glue it down.About every 16" .Run a line of glue along the edges.That should keep it flat for you.
You can get free cardboard from appliance stores that sells washer & stoves & refrigerator boxes. THAT'S MY 2 CENTS:rasta:

:dance: