Aluminized mylar, like in a party balloon, has very low permeability for all gases (even helium). I use them for anti-suckback during cold crash, and they hold enough gas for that. The balloon can be folded flat and rolled to get almost all the air out at the start. The balloon is in place for days, so I use stainless steel tubing and double-wall EVA barrier tubing to reduce any oxygen coming in through the connection hoses. The balloons are small and it would take several to hold a 5 gallon keg's worth of gas for closed transfers, but it could be done. For a small fermenter, as in the photo for the original post, the balloon should be big enough. I use a keg instead, and use the fermentation gas to purge it. That kills two birds with one stone.