Here we have two kinds of window screen replacement corners.  The solid corners are stronger but require the screen to be partially disassembled for corner replacement.  The 3-part interlocking pieces are used to replace a broken corner without removing the screen.  

These aluminum frames use imperial dimensions.  The inserted part is about 0.38" tall x 0.68" wide in the rectangular opening.

I have made them out of PETG printed with 0.2mm layer and QUALITY settings.