Gas storage bags could be made of different materials. Their size varies from 0.5 to 10 cmu (cubic meters). Smaller sizes are pretty unusual. There are several different variants of materials and sizes available on the market.
Most of the bags are made of reinforced PVC. The PVC gets sealed on the sides so that it is airtight. They usually have one connector sealed into the PVC where a hose or pipe could be connected.
The German company B-Energy created a biogas bag rucksack. The rucksack is two-layered. The inner layer is made of PVC, that is the actual gas storage. The second layer protects the inner layer from damage. Additionally there are stipes mounted onto the outer layer so that people can easily carry them like a rucksack / backpack