To be more precise, for the rear bulkhead, do not built according to manual step. Built the buklhead outside the chassis together with its brace. Then put on top of glass table and see if bulkhead sits perfectly (for sure it wont). Use you hand to manually twist between L and R bulkhead to make sure they sits perfectly on glass. Then mount to chassis and start putting screws one by one and tighthen in `star` pattern.