When I think of bags, there are two things that come to my mind. Either using the same mechanism as inert gas method meaning that a tube constantly pours gas into the bag from the reaction chamber/gas cylinder depending on the gas you use.
OR, you fill the bag with gas before you tape the bag over your head/on your neck. Most probably you meant the first one because I can't see how you can put a bag filled with gas before you leak/spill the contents in it to your room before you can seal the bag on your neck.
Making a constant flow appears to be more complicated and prone to errors way to me. When you prefill a bag with a gas, you can easily estimate how much effective gas entered inside and the approximate initial concentration you have there. Besides, sealing the bag cuts off access to atmospheric oxygen from the outside, reducing miserable chances to survive even more.
Initially I was thinking about using a single bag prefilled with a sufficiently big amount of gas, assuming that a great part of it can be safely sacrificed on escaping from the bag at the moment of pulling the bag over the head. Recently I reconsidered my protocol for H2S poisoning (which can be adapted for CO or HCN), introducing two inner plastic bags that are supposed to be nested inside the third bag.
The first small bag is used to collect the poisonous gas. When the bag is filled with enough gas, it's detached from the vessel with reagents (or the filter if the gas is washed) and placed inside the second small bag (open side to the bottom). Then the second bag is placed inside the third, bigger bag (bottom to the bottom) and the outer bag is placed over the head. After that, the outer bag is sealed around the neck.
Once the first bag containing the gas is detached and becomes open, all further operations mentioned above should be done quickly.
This technique aims to reduce the amount of gas escaped to the ambient atmosphere and minimize contacts of the skin with reagents that could enter the collecting bag in the form of small sprays during collecting the gas (this is relevant for gases obtained without further filtering).