Use this Corner Bracket for 2020 Aluminium Profile to connect and cover two lengths of 2020 profile at a 90° angle.

Print at 0.08mm layer height with 50% Gyroid Infill for the best results.

The structural load is borne by the shape, not the walls. Once you connect 4 of them, it's quite strong.

Once you print them, dry fit your parts and then whack in with the base of your palm. No need to use a mallet or anything stronger.

Inspired by Gabe! https://www.youtube.com/watch?v=NLeTvSaPJIs