I recommend to print with 4 perimeters in order to have the little hooks solid since the infill in them is so minimal. 0.2mm layer height.

This is not my specialty, however my girl friend needed an organiser for cross stitch and this was a simple cheap option instead of paying 12€ for something. This is after several revisions.

I had a 1mm gap for the openings originally but the string kept slipping out now it is 1mm wide but then at the initial opening it is 0.5mm so that it doesn't just slip out.