I think for printed parts, you might have to add an insert for an M3/M4 nut, as I reckon the thread will strip too easily...
...if Serek already has a product that he's selling, I will leave to him, as he has a business to run.
I might be wrong, but I have the impression he wasn't trying to do it on a large scale, money making venture. Not sure of the details, but I suppose it might be courteous to have a chat with him before printing off xxx number of them.
I have enough to do TBE should they fail (and that already has ali ones), so only even looking at it purely as an exercise. If it looks OK and seems durable, then if there is the possibility of the shop helping to support the site, that's fine. But if one of our sellers/repairers has a commercial interest in selling the same, I'd rather see that