Imagine a 5/8"(16mm) cable running through 6"x6" (14cmx14cm) pressure treated blocks about 4 feet (1.2 meters) apart. The cable gets wrapped around the stack and connected to itself through a turnbuckle which is tightened to provide tension.

Now think of a steel right triangle with the legs being the vertical and horizontal aspect with the hypotenuse supporting the horizontal leg. The horizontal leg has a vertical tube welded where it meets the hypotenuse. The vertical leg as a hook curving down from the top away from the horizontal leg and a wooden pad at the bottom. We called these stackjacks but a Google search tells me that may have just been what we called them and not their actual name.

The hook hangs on the cable discussed above and the pad rests against the smokestack. 2 2"x12" (4cm x 29cm) walkboards are laid between these stackjacks. The vertical tube at the end accepts poles to which a railing is attached.

TLDR look at these photos of a company that does it exactly how we did. https://apexchimney.com/wp-content/uploads/2024/02/exteriori... https://apexchimney.com/wp-content/uploads/2024/02/IMGP40400...