I designed a captured nut mounting system for a workbench tabletop. I wanted something more secure than a z-clip so that I could pick up the heavy bench from an edge and not worry about it breaking or slipping. My bench weighs around 200lbs when picking it up from an edge, and the brackets are holding it with no signs of failing.

Use either ¼ or 5/16 diameter bolts, 2.5" long, with a washer. Then end of the bolt shouldn't protrude from the top of the bracket. I designed this with 5/16 in mind since it is what I had on hand.