So I finally got around to running a second experiment. This time I opted for a 6" system with various sorts of linings.
First, the heat riser. I cut another batt of the Roxul into 9 pieces, each 23" long, each with a trapezoidal cross section and a short face of 3". Easy with an appropriate cutting guide. After cutting three pieces in half, I had the parts for a hexagonal riser close to 3' long. My wife mixed some powdered clay, water and furnace cement, achieving a "light buttercream consistancy," and spread it on two sides of each riser segment. It was a piece of cake, for a baker. I may eventually go into more detail, but this is something that, as I expected, did not work out well in use, so the details aren't worth much attention.