seems about right. i would suggest to tape the tubing on the backside of the bag wuit high up. that makes it comfortable and the inert gas whichever you use will push the co2 down. would not recommend argon though because it has almost the same density as co2. argon 1,4 and co2 1,5. its is still up to you because i have read about successful attempts with argon. you have a regulator for 20 liters that should be enough anything below that would be risky.
how did you construct ykur bag if i may ask?