1. Cut a hole in this part, right here. Thats why I added the highlightet lines.
2. Added a Cylinder, smooth shaded, and auto smooth.
3. First problem arrives. If the cylinder only intersects the upper part of the object I get artifacts. If it scale the cylinder in z direction, so that it intersects the whole object I dont get artifacts, so I do it this way. I also use the "Fast" Solver as this also does not result in artifacts.
4. I apply the boolean and there is my hole. No artifacts. But when I extrude it down, artifacts start to show up. I tried adding supporting lines. I tried extruding the inner faces of the hole and scaling them down. Face normals are correct. I tried "clear costum split normals data" ind the object data" but nothing worked.