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Human fuel only by the look of those. I ordered a couple of the 1.5L rectangular ones linked above. Can be neatly strapped to the inside of the transom on my SIB for excursions up river. Very tidy solution and just right size for a refill,I'm pretty sure you can buy fuel bladders. They come in different capacities and are so compact because they are basically vacuum sealed bags that can lie flat, Maybe an option.
Ronhill 1.5L Fuel Reservoir Running Marathon Large Filler Cap Refuel Bladder 5051508478070 | eBay