This bracket is designed to prevent a simple 2020 and 2040 corner from twisting when it is joined using a single corner-bracket. See illustrations.
Please post a make, and share your findings, or at least a like if you find this item usefull.
Colorfabb, rigid.ink etc. PETG or stronger
Experiment with print-height. It needs to be a close fit.
Support should not be needed, but will depend on printer.