i made one out of 3inch or so box section , going into an old fire extinguisher insulated with glass wool surrounded by a propane cylinder leading to an alloy exhaust, was allways a devil to light but once going the propane cylinder would be glowing red hot after only 20 minutes, i copied it from one on you tube ,fitted a cheap woodburner heat dial and it read 700 degress quite rapidly ,never had the bottle to take it inside the shed though & its still sitting outside unused .I would have uploaded a photo but dont know how too.