I printed out the WK9 improved extruder for my coworker who had followed my lead and bought the MPSM. He brought the printer to work and we installed a glass print bed and then when I went to install the WK9, I looked and realized he hadn't gotten the M6 nut like he was instructed to do. Well, I thought we might be sunk, but then realized I still had my printer. I fired up Fusion 360, imported an M6 nut from McMaster-Carr, then added a little extra bit just in case I needed to pull out the insert. Exported to Cura, printed solid and it fit perfectly. Might not be quite as durable as a normal metal M6 nut, but it'll work for now!
Could be printed in any material you want, it's not a high stress part. I do suggest printing it solid, or at least with several shells. Even solid it's a quick print.
I've found that the sweet spot for printed threads is roughly 1/5th the thread pitch - so there's no reason to believe you couldn't print at 200 microns (0.200mm) and have excellent results.