Well, I don't have one handy, but the opening is just a tiny bit wider than the outside edge of a horizontal shelf-mount frame with a slot on the right side top and bottom for the bottom frame flange to slide in. Then there are two bolts mounted above for the top side flange to slip onto, and two holes on the bottom for bolts/nuts to slip through to hold the bottom side flange in. There is a piece of wood inside below the frame for the bottom flange to screw to as added support.
Perhaps that will help you visualize a way to make one...