This is a compact 90 degree brace for Openbeam/Makerbeam XL 15x15 extrusion. There are two versions - one is a straight 90, the other is a very handy 90 degree rotated variant so you can do crossovers. The crossover version is pictured in the as-built photo.
A couple of things make this design different than the numerous other corner braces out there. This one has channel grooves to keep the part stable, which makes for a really nice, sturdy brace. I also did countersunk holes to make for a nice clean installation.
You do need to print this with supports but cleanup isn't bad.
Supports are required. Rotate the part so it sits flat.