Would you still have the problem with CO poisoning if your burning denatured alcohol?
Yes. Any incomplete combustion will produce loads of CO.
Canoedogs - I like your setup. But then again, I'm a stove fanatic, and fully aware of the CO poisoning in poor ventilated setups. It is good advice to maintain a good distance between the flame and the cool cocking pot - cooling the flame by putting a can of snow inside it is an excellent way to produce lots of CO. There are a lot of camping stoves that could benefit from raising the pot a centimeter or two.
I am guessing the reason for the "box heater" setup is to have some sort of physical boundary between the upholstery of the car and the naked flame - or is there any other reasons for it? A naked flame would heat the car just as well or better, but could be considered somewhat stressful in that setup.
Any tips for the most important aspects of the design or use of box heaters? Although I'm an old stove fanatic, box heaters are new territory for me.