sometimes seen on the drop-down part of trucks ... it has a lozenge
shaped plate of metal with a cutout in the centre which is fixed into a
bolt which screws into the substrate. The flap has a hole which this
faster passes through, and you then slide the metal plate which drops
down and locks the flap in place.

Can you get small (c. 5-10mm ?) ones for a DIY project to screw into
wood ?