try looking for the parts bag with the battery cover, it's really cheap and easily available, you also get lots of parts that usually break from that particular parts bag like uprights, shock towers, arms etc. regarding the body stiffner i dont think it will help with regards to lengthening the life of the spur gear cover, for one thing it might even add additional stress on the spur gear cover because it is linked to it,,,
one good practice though would be putting thread lock on your spur gear cover before you place it on the chassis, when using a really hot motor like the novak 3.5r the spur gear cover tends to lift up from the chassis because of the screws not having enough bite on the main chassis causing premature breakage of the spur gear cover.