I'd thought for a while I should try out using heat inserts, but seeing insert presses and not being sure about melting stuff into plastic was even terribly practical it stayed on my list of things to checkout for far too long.
My first use of them was for 8 mounting posts for a PI-5 and SSD board. Measuring the M2.5 insets came to an outer diameter of 3.5mm. The inserts themselves have a smooth surface at their bottom with 2 bands of rougher knurled (by the looks of it) texture.
I decided to go for a 3.5mm hole for the inserts. This might seem counter intuitive, as a common practice is to use a hole slightly smaller than the outer diameter of the insert to ensure a tight fit. However, using a 3.5mm hole meant the wider knurled section of the insert was slightly larger than the hole itself, which is what provides the grip. This also had the added benefit of allowing the insert to sit just inside the hole on its smooth bottom band, ready for the soldering iron to be applied.
I bought a kit that came with a "digital" soldering iron - this is key as it had a temperature setting, I often print PETG quite hot, (250-255 C) so I set the iron to below this at 240C
The tools for the end of the iron weren't up to be much not staying on either the iron or the insert itself so I opted to put a bolt into the insert and use the iron on the bolt.
I found that a gentle downward pressure allowed the insert to slowly sink into the print, as a bonus the flat head of the bolt made it very easy to ensure that the insert was vertical.
You can find the stl and blend files here https://www.printables.com/model/1374312-pi-5-and-ssd-base-plate
enjoy!