I was just in need of something to construct a lightweight box for my laser engraver.
Local hardware store has those PE square tubes that can be cut easily but have to be connected. So i tried to design a corner for those.. and failed..
Since I still needed something to create those tubes I went with a different approach. My corner consists of three parts or rather two different types of parts that can be connected in different ways to create a T or corner shape.

It's modeled in FreeCAD. I use a varset to modify sizes. Mine are 8.5mm inside, 1.5mm wall thickness and 25mm connector length
I added a file which contains all 16 parts to build a box with those tubes.

Supports aren't needed.