Hi Pete, have the tundra 4 (the polycotton one). I used one of the tentipi large fireboxes in it last week, and it was fine. Even with the wood being a bit wet (a tad smokey), non of the sparks that spat out came near the fabric of the tipi. They do heat the tipi up a treat and are easy to cook on. Only draw back over an enclosed wood burner is that i would'nt fancy leaving it burning over night (am working on a lid so that it can be closed down safly at night but still retain some heat).
Baggins