This Caster Corner, for lack of a better term, was custom designed and built.
p1.jpg
It is made of 1/4" steel plate & angle iron. As you can see, it fits the 2x4 nicely. Countersink the holes and the platform lid sits flat. p2.jpg
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. p3.jpg
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 "TreesT, Drilling holes in the sides of the metal holds these 2x6 pieces square. p4.jpg