Customizable square tube corner bracket

This corner bracket is designed to connect square tubing to make a box shape. It slides into the square tubing and has no holes to attach to the tubing, so it's a friction fit or glued in.

I designed this for use in my [perpetual pendulum wave project](https://github.com/jbeyers/pendulum_wave). At the time, the only corners I could find were printed on their side, which had 2 problems:

* They needed support and/or post-processing
* The print layer orientation made one of the posts weak.

This design avoids both those issues. The orientation of the layers make it so that a single layer extends into all the tubes, which strengthens the part. It prints without support on my very, very homebuilt printer.