bondo shouldnt be relied on for structural integrity, unless its kitty hair of courst. i noticed you are using fiberglass cloth. though its a good choice and much easier to use, it is not nearly as strong as fiberglass matte. when using cloth as an alternative to matte, i like to use about 50% more cloth than i would use otherwise with matte. try adding a few more layers of cloth. then test the strength. if you feel that your amount of layers is already sufficient, then, like previously stated, reinforce the flat parts with 1/2 in. MDF. hope this helps.