A cover for a structure may include a plate including upper and lower surfaces. A rim portion of the lower surface may be configured to contact an edge of the structure. The cover may include a spider including a center portion, a plurality of legs extending radially outward from the center portion, and a plurality of coupling elements. Each leg may be coupled to the lower surface of the plate with a corresponding coupling element. The center portion of the spider may be movable away from the lower surface of the plate to move the spider from a locked position in which a distal end of each leg extends outward to a first radial position to engage the structure to an open position in which the distal end of each leg is withdrawn to a second radial position radially inward of the first radial position to disengage the structure.