The main profile and the sides are produced much the same as for the simpler external chimney (above). The main difference is at the top of the main elements, and the presence of two additional pieces (a separate back for the main flue and a tube for the secondary flue).

Make a front profile as for the simple external chimney (above). Then produce a side element like the external chimney above except at the top segment. One of the sides (below left) will be cut vertically, leaving a section that should be split horizontally by a fold line where the height (a) matches the width of the chimney. The other side is trimmed to leave just the width of the primary flue.

The back of the top of the primary flue is finished with a small rectangle with tabs. It is the height and width of that segment.

The secondary flue is just a series of sides like a roof chimney with a simple straight bottom edge.

The back profile omits the top segment. It does not need to have an applied graphic design because it is essentially a big glue tab that attaches to the side of the building. No part of it is seen.

1. Side
2. Side
3. Front
4. Back Primary Flue
5. Secondary Flue
6. Back (no graphic)