Joining a flat surface to a Cylinder

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: