create a cylinder.
Add it to one part and then at the same place on a different part, difference a cylinder with 0.5m larger diameter.
Probably only needs to be about 10mm long.
Getting a precise match up is dead easy in openscad.

What i would suggest is that you design a few simple things first. You need to get into the right mindset for openscad. Once you've got the basics, you'll see how easy this kind of thing really is :-)