The trick here is that @DaveR is making use of the ‘stickyness’ of SketchUp, nothing grouped, yet. Once placed, the edges merge. To make the mitered details, the Follow Me tool is appropiate, select the path, first and then activate the Follow Me tool, then, select the face:
Here, I use Shift to extend the selection and ‘Offset’ only three edges.
It is wise to draw a face in the right plane (Press Arrow keys for direction) Then draw the outline ‘on Face’
Still, everything is plain ‘raw’ geometry, nothing grouped.
For the actual mold, the same thing aplies, draw a rectangle to draw the miter and chamfers:
Again, nothing is grouped: Rotate the selected geometry of the mold, activate Move tool and pick the center to move it to the right spot on the chimney.
Erase the edges (those lying on the ground were ‘internal’, so they will wind up in the mold as well…)
Right-click on a face that has the right direction (White colored, my back face color is red, @DaveR 's is green) and select ‘Orient Faces’
(You can now group the mold…)
chimney.skp (430.5 KB)