3-way square tube corner

I wanted to build an enclosure for my 3D printer, and I wanted to use aluminum square tube stock as the frame. It's designed to print without support.

The included STL is for 1 inch square tube. It mates with this stuff:
https://www.homedepot.com/p/Everbilt-1-in-x-96-in-Aluminum-Square-Tube-with-1-20-in-Thick-802537/204273939

Since I was going to adhere panels to the outside of the box, there's no mechanism to lock the tubes to the corner. The source Solidworks file is attached, so you're