im thinking maybe tapping a garbage bag around my head and using ducktape to create a seal or using an oven bag.
That can work. Rapid loss of consciousness, comatose state, and death occur at concentrations of oxygen of 6% or below. It would be best if no more than 1/4 of air were mixed with helium after pulling the bag over the head (residual air in the lungs should also be counted), assuming that the initial concentration of asphyxiants you have in the tank is 99% or above. In order to increase the absolute critical limit of air in liters, a bag of a large volume like 20 - 40 liters should be used.
It's very easy to check the possibility of having 80% He + 20% O2 mixture just by inhaling the gas from a latex balloon or a plastic bag through the mouth. Before inhalation, as much air should be exhaled from the lungs as possible by applying a significant effort. Then the tested gas is inhaled contiguously till filling the maximum capacity of the lungs. After that, breathing should be stopped and the person who carries out the test should lie on their side on a bed in order to avoid injuries due to falling. If clouding of consciousness and dimming of vision occur within 20 - 30 seconds since inhalation is done, it's a good sign that you have an asphyxiant gas of a decent purity. Otherwise, you probably got a wrong gas mixture. Breathing may be resumed once either clouding of consciousness or a strong urge to breathe occurs.
