It is made of 1/4″ steel plate & angle iron. As you can see, it fits the 2×4 nicely. Countersink the holes and the platform lid sits flat.
The holes in the angle plates are to screw into the bottom of the platform’s frame. A hole is drilled into the side to allow another platform to be bolted to this one.
Here it is in action. This particular frame is part of a tree base being built for Into the woods . The lid is being left off so a technician can be inside the tree to move it on & off stage. See the page “Trees”, Drilling holes in the sides of the metal holds these 2×6 pieces square.