I'm using liquid nitrogen much easier, cheaper and lethal.
Liquid Nitrogen LN² cost $1 per liter and expands to nitrogen gas at a rate of 1 to 700, so 1 L of liquid nitrogen will evaporate quickly and displace all of the oxygen as it expands to nitrogen gas at 700 L of N2 gas. What is more, is that LN² not only has killed people in industrial accidents due to leaks because it sucks up all of the oxygen or displaces it, but it's used for nitrogen purging which is like dessication - breathing it in at such cold temperatures would condense the oxygen out of your lungs and purge atmosphere of oxygen.
You don't need any fancy equipment hoses or valves, just a cryogenic Dewar tank. You can get one from Alibaba or Amazon for $150, but you would need cryogenic gloves and a face shield.
10 L of LN² will expand to 694,000 L nitrogen gas displacing all of the oxygen if left to evaporate with a heating element and in a small vehicle cabin will replace the whole atmosphere or pressurize it since 694,000 Liters of N2 gas is about 30 cubic meters and that's about the size of the interior volume of a vehicle's cabin.
You just have to make sure that you leave a little Gap open for the interior atmosphere to escape and don't get frostbite, get a sleeping bag or something and you have to pour hot water or something on it so it starts to evaporate. It's a painless death and industrial accidents have occurred where people have died within a breath or two without any sense of panic, and then someone goes into rescue them and they become another casualty, and then third rescuer becomes a third casualty and then like casualty accident because the body won't not sense any suffocation due to the hypercapnic alarm system not recognizing nitrogen is a threat since 78% of our atmosphere is composed of it.
There are of course many other details that are important to discuss in terms of procurement and the rate of evaporation, the choice of the confined space, and other variables that I may make a method post on that I've been procrastinating for the past year. But I've tested it in my vehicle using a pressure temperature and oxygen meter inside and using a heating element the entire cabin was filled with with cool vapor gas and the oxygen level sensor alarm was at 1.5%, where 3% is instantly lethal, while the temperature below freezing. This was a 10 L Dewar. So if you take something to knock yourself out like bensos or you're just completely knocked out for the whole two days on an overdose of Benzos / opiods you'll definitely die of hypothermia if the method fails or instantly from inert gas.
Other candidate confined spaces include small closets with warning signs to not enter, one person pop-up tents, regular tents, DIY coffins where a heat exchanger ensures that it doesn't turn into a freezer, possibly one of those hyperbaric oxygen chamber makeshift non-solid Chambers that are like massive sleeping bags, small compartments, elevators and the one rule about transporting liquid nitrogen is you never transported in an elevator because if it spills you die instantly, hotel room bathtub and then turning the hot water on to make it evaporate.
Because the pressure will increase substantially not only atmospheric but the vapor pressure If you studied physics will cause a nitrogenized environment and so you don't need any CO2 scrubbers because the density and ratio of nitrogen entering your lungs and the fact that it is also condensing the oxygen out of your lungs won't make a difference how much CO2 your expelling, although you're that paranoid you could get a CO2 and 02 scrubber chemical I don't know what the name is but it's some powder that they use in operating tables and surgeries. Very cheap stuff very cheap stuff. Very easy to get in the US at least, you just go in with your tank you're a paying customer, they're a business.
If you're paranoid then go in with a liquid nitrogen ice cream recipe and tell them you're making liquid nitrogen ice cream. It was even a YouTube video If you Google how to get liquid nitrogen of a guy transporting it illegally in a car and the company's letting him with the two kids inside, Jesus Christ they could have all died and the guys filling up the tank or all just like all right it's 35 bucks.
Probably not applicable in the UK but not impossible, you might have to open up a small tiny business or website on cryogenic cooking or telling them that you're using it to micronize coffee for espresso and they'll sell you small quantities but you might have to travel or rent one of their big cylinders and have it exchanged, but if it becomes a popular method they'll be remiss to sell LN² freely and become more restrictive / selective.
AFAIK you will have no issues getting LN² from welding shops, industrial gas suppliers, ice cream supply shops, and chemical supply depots including of course your local University chemistry stockroom. In the US and most parts of the world, although the UK is a different animal.