I have a sedan-sized car that figures into my plans regardless of the specific method (ie... one could exit bag in the car as well.) It's in a small garage with a closing door for privacy. I liked the idea of the liquid nitrogen because it combines the method of inert gas without having to use any headgear.
Sort of like a DIY sarco in your beloved car. Sounds about perfect.
Edit: I might be worry about some it leaking out though. It seems crazy strong and totally undetectable by any passersby. Might not be very responsible unless you had a totally sealed chamber/very secluded location. Although, the cases of accidental death seemed due to continuous leaks, whereas a few liters would probably dissipate rather quickly. The fact the the car would be enclosed within another enclosure -- the garage -- though not airtight, would likely prevent a large buildup outside. As long as one didn't use much more than one needed.
Liquid nitrogen is not straightforward to obtain, it turns out. And it has to be transported in some kind of special vehicle. Maybe delivery is possible?
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Cost wise seems good. The container is what will cost you a couple hundred bucks.
See: wikihow.com/Purchase-Liquid-Nitrogen