They may not be very sexy but what is wrong with the 5gal plastic water bottles used on coolers and available everywhere?
Think about the size and weight then reply again.
This was suggested to me by another contributor, im terrible about finding thing here
These 10L bags are tuff and fill easily. Best they fit anywhere. I will be now traveling with 30L that fit easily behind my seats .
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They weigh less than a pound empty and the snout extends maybe 4 inches above the main tank. Other than a bladder what other vessels weigh less and take up less space? What other vessels are cheaper or easier to find? What other vessels can be easily exchanged? If you are concerned about durability than sleeve them in the plastic crates.