A screw clamp for a gapped wall such as a parallel-rail side wall of a baby's crib is designed to include a block with a screw extending from one side to pass through the gap in the wall, and a knob threaded to mate with the end of the screw emerging through the gap. The knob is reversible, with a threaded recess on one side and a protruding hollow shaft with a threaded interior on the other, to accommodate a wide range of wall thicknesses or rail widths.