If you've got a need for some additional frame-building parts, you can find all of my various 2020 extrusion builds here.

Just a little 90 degree elbow. Configured for at least two M5x10 fasteners per extrusion (can be three on the long side if you also add the one in the corner, as shown in the images.) I printed mine in PETG and each takes about a half hour with a 0.6 nozzle at 0.48 layers.