I remember back in the late '40s using a balloon for a tank. You had to have a hatch so you could get at it and get the air out of it. Then if it broke you had fuel all over everything. If you crashed you would get fuel on a hot engine and likely have a fire. In the early 60's a lot of p[eople used a plastic bag. Same problems. Then when the clunk tank came out it was much better. You could put your tank in and forget it.
I don't think I have ran across anyone using a baggie in the last 40 years.There must be some reason for that