Square Extrusion Clamp

This clamp joins [OpenBeam](https://www.amazon.com/gp/product/B0196FGT0Y/ref=as_li_tl?ie=UTF8&tag=mgx3d-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B0196FGT0Y&linkId=56ad65afeb7fa366f687274163afd791)*/Misumi 1515 extrusions at a 90 degree angle.

It is meant for axial and radial loads, it will probably not do a good job for angular loads.

The screws (M3 12mm) are off-center by design for two reasons:
1) Make it as small as possible
2) Prevent stress/cracks al