The triangular stool, which I have made in 2011 features in a 'do-it-yourself' article in the latest issue of the German re-enactors magazine Miroque. The complete issue 11 of this magazine features many short medieval ideas/guides of making your own stuff. Although I do wonder if you actually can make the triangular stool using the sparse 2-page guideline in the magazine, it does provide the more complete sources (this website and that of blood and sawdust) that will help you make the stool. From this issue I did get an idea of making a clay oven, to use for medieval cooking, but that will be a project for later.
The article in Miroque.
The triangular stool, one of the five images used in the Miroque article.