I needed a corner bracket for the top of an enclosure I'm making out of 3030 radiused aluminum extrusion. I couldn't believe the prices of some, and others came with square covers for a radius bracket which made no sense, so I designed this to be a simple way of attaching the pieces. I have done stress tests and printed in PETG with 4 walls and 60% gyroid infill it took a bit for me to break it using leverage on the ends of two 24" sections bolted to it. I think it will work well in stationary builds. This enclosure is free-standing and all the other joints are blind machined in joints which will carry the brunt of any shear forces acting against the side of it.

This joint requires the use of low-profile socket head screws, available from Mcmaster-Carr (https://www.mcmaster.com/93070A177/) and Fastenal (https://www.fastenal.com/product/details/90601). I haven't found an alternative that has the right dimensions (13mm x 5mm head) but they aren't that expensive for what they are, hopefully you have other things to order along with them.