TubeGrid

Metal tubing is a great way to assemble frames of all kinds. This connector is designed around 1/2" EMT (U.S. electrical conduit) which is about 18mm O.D. I've included the Fusion 360 file if you want to customize it. I used a 1/4" x 1" thumbscrew and nut which is installed into a hexagonal recess in the body of the connector. It does require support, but I found that after removing the bulk of the support with pliers, just pushing the tubing through the hole cleaned out the rest pretty well.