Here is the end result we are aiming for:
It lets us connect the top of a cylinder to other flat brushes, allowing us to make a round hole in a floor or similar. It doesnt matter if you are using a cylinder, or an inverted cylinder, this should work for both.
Method:
First off, because radiant doesnt know how to cap a cylinder, we need to make a bevel.
Make a brush around the lower right corner of your cylinder:
Now, go into the "Curve" menu, and select bevel:
now cap it like so:
Delete the bevel and the top part of the endcap:
now move the bottom part up to the top and copy/paste/rotate it around:
Add brushes if you need it to be wider:
