Or maybe you can buy some thin steel sheets and glue it (with industrial glue) to the plastic cover. I don't know how his would affect the electronics but it would definitely add some weight to the body. Then again, you can buy a new plastic cover and glue some steel to the "outside" of the cover. You don't have to damage the original cover and you'll be able to add much more weight than if you tried to glue the steel to the "inside" of the cover. This will also prevent any issues with the electronics.