Abstract: Apparatus for drawing taut a cover sheet over a box opening such as a truck box or a walled building of rectangular plan form provides stop members mounted on a longer sidewall presenting stop surfaces spaced below a longitudinal edge, the surfaces forming a socket for captively seating a tubular or bar shaped roller to which the sheet is tangentially attached. The sheet is fully unwound from the roller in covering the opening, and an overhanging sheet portion is tensioned by a fractional turn of the roller additionally in the unwinding direction, causing the roller to press upwardly against a stop surface engaging the top of the roller and to press inwarldy against a stop surface, with the overhanging portion disposed as a rain-shedding eaves maintaining a seal at the longitudinal edge and terminating under a clamp tangent to the roller.