The instructions to fold for the octagonal box can be found in the book Origami Boxes from a Single Sheet, ISBN 4480872035, page 58. In the original instructions the paper was divided into 9 sections, with one overlapped section.
Instead, to make the mold sturdier, I divided the paper into 12 sections, cut one off, and overlapped 3 sections. As you can see in the picture above, the mold did the job very well and did not unfold at all. I also modified the height of the box to get a mold of about 5" (125 mm) tall.
I used a 30" x 15" (750 mm x 375 mm) rectangle of Reynolds Genuine Non-Stick Parchment Paper (27½" x 15" after I cut one section), which, by the way, it's a very nice paper for folding.