Extract points from the area of the surface if the imported model to create a plane using the Model Work menu.

When you extract points, it is best to explode the model so that you only extract limited surface area, not the entire tube - which may generate too many points for VTube-STEP.  (STEP can handle up to 10,000 points on-screen at one time.)

In Create Plane from Point Cloud (Space Objects), select the points that describe the shape of the plane.

In select mode, click on the first corner point, then hold the shift key down and click on all the rest of the corner points.

Then click on Test Build. You will see something like this.  Be sure the plane hull (boundary) looks correct, and the normal is moving up.

Create a NEW plane and then press F2 to redraw.

Use Constructed BOSS: From Space Object to create a boss from the new shape.

Test Build, then Create NEW

Drill with the boss.