This is an aluminum extrusion 90 degree connector we made for motion control rigging a diaper for a commercial shoot. It is a very tight fit so a mallet may be needed when inserting the corners into the aluminum extrusion.

The specific aluminum extrusion is the Mitsumi 25mm x 25mm, 5 Series Base 25 linked here: https://us.misumi-ec.com/vona2/detail/110302685050/

The aluminum and this print have a hole sized for a quarter twenty screw.

We did not have a large payload while using these connectors so I cannot guarantee they will work for your purposes.