I have created this little corner as a replacement for the mosquito screen corners as available in various (Dutch) hardware stores. In the original bag there are six (!) corners. Not 4, not 8 but six. Ridiculous. 

Anyway, had some broken screens as well so created this in FreeCAD. The part fits nicely. Printed with supports and 50% infill for strength and in white PETG. 

Update: have uploaded 2nd version with a bit more knurling / grip for tighter fit. Start with version 2 and if thats too tight try the first upload.