A trailer has a container with top doors having a frame and weatherproof thermoplastic inserts. Sealing flaps are provided at edges of the top doors to make sealing contact with sidewall top rails or top edges of end walls and tailgate. The top doors have angled extensions with grooves for ball gaskets to overlap in hand-in-hand fashion to form a reliable seal where the top doors come together. The system provides a weatherproof means to enclose the container to prevent or deter entry of water.