The wedge has been around for ever. It will fit your needs.
As to fire retardant.... Shop the makers..... most all offer pre treated fabric. Californackstan has the toughest rules, go figure. Be sure your basic fabric is "Army Duck" weave. It is heavier with double threads in both the warp and weft. Also most makers are using a highly water repellent fabric that starts out as Army Duck. The repellent is applied at the mill and the fabric we used really worked as long as you did not spray an aerosol bug spray on it.
EDIT: Regarding fire retardant.... there may be a bit of a hang up depending on where you do most of your camping. Usually materials with fire retardant from the manufacturer have a seal or code or whatever that states the fabric meets fire retardant specs. If you apply the material yourself there is no certificate. This comes from some conversations I have had with folks in the past three or so years and really only applies to states where fire retardancy is required when camping on some public campgrounds.
Wife and I were one of the five companies back in the day (Terripan Ridge Lean to and Tipi Co.) building tipi's, tents, lean to's and wife designed the original Lean Pe. We used over 1000 yards of canvas per month, many months.