For the under-bench stowage shelf I found things slipped off sideways or at the back from time to time. This is the first time I’ve added an upstand to prevent this from happening, though I have enclosed sides and back on many workbenches. I don’t want an enclosure this time. I may at some point want to pass long lengths under there for whatever reason so what I added can be folded in at each end just in case.
I used a 3″ butt hinge in the corner, fixing it first to the ends of the side pieces on the inside faces as shown and then, flushing the side piece with the end of the back board, fixing the hinged end pieces to the back board.