This 9mm pitch roller chain prints up quickly (about 2.5 min/link) and assembles easily. Most importantly, the links are designed so that the tensile stress is aligned with the print lines (rather than across the print lines) for maximum strength. When printed in PLA (recommended), the theoretical yield strength is about 22kgf for the chain, although a more realistic working load would be about 5kgf.

I've included both STL and STEP files for the links and for a variety of sprockets (8 thru 30 teeth).

Print using 0.2mm layers. I would recommend PLA over PETG, but glass or carbon-filled filaments might be even better.

The links just snap together by flexing the ears apart and slipping them over the nubs on the mating link. The fit will be tight at first, but pinch the two ears together as you work the link back and forth, and the two parts will smooth each other out.