We have 2 of the Pro-com flame heaters from Northern (northerntool.com). They work really well and are cool on the backside, so you can get them fairly close to the canvas without danger; the front is hot! We have a small 10,000btu that doesn't do for really cold weather (below 20 deg), but the 25,000 btu does fine, and it has a thermostat on it. The most fun is the propane firepit (we have a tipi) that puts out 60,000 btus, but is not thermostat controlled, and isn't as safe as the wall heaters.
They all take standard fittings, but do require a regulator (an extra $30 or so for a good one).
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Don't take life too seriously, it isn't permanent
Soaring Spirit