Instead of screwing into plastic, we recommend strong and reusable machine screws and heat-set inserts.
Heat set inserts are only reasonably priced and have a grooved outside, with theads on the inside. Originally they were designed for injection molded parts but they work fabulously for 3D printed plastic as well.
You could use a plain soldering iron to try to heat-press these in - Heat up the iron and use it to press the insert in, wait for it to cool, then use like a hex nut that's embedded inside the plastic.
https://thinkrobotics.com/products/brass-heat-set-inserts-for-plastic?variant=39635373686870
Available Quantity : 787
Package : M3