Corner Brace for 13mm or 1/2" square or angle stock - useful for quadcopters

I wanted to make a quadcopter that was stronger than the usual. I had an X525 and I wanted to add longer legs. 13mm square tubing was the obvious choice, but there was no good way to attach them. These corner braces work well for that purpose - and a lot of others.

This brace works for 1/2" stock as well as 13mm.