4 inch flat 90 degree interior bracket by THEOLDGOAT on Thingiverse: https://www.thingiverse.com/thing:7296930

Summary:
4 inch flat 90 degree interior bracket for benchwork under model railroading table. This bracket was designed for a 4x8 model train table as a connection point from bracing board to the side boards.  Fits 1/2" width ply.  Use kreg pocket screws for attachment.  