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Am I going crazy, I thought I answered this this morning, but I'll do this again. Depending on the size of your shack you can get a propane heater made for travle campers, they are perfectly safe & you can put a vent in the top of the shack just like the campers. I'm sure if you go to a camper supply they can advise you. You can also get kerosene heaters as well. A lot of heaters have CO sensors, if not you can get 1 @ Home Depot. As for seats, I would put bench seats then you could use them to lie down & take a few zzzz.
I have been looking for a while and finally found some that weren't too expensive. A lot of the ones I had been looking at were around $25 each. I found these shipped free from good ol Communist China and they seemed pretty reasonable. http://www.ebay.com/itm/170877498442?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649