This idea is not under it, but you can get an Over-the-Ladder chair hangar from Camping World and put it on the ladder. Take your tank and sit it in the hangar. You may have to bend it depending on what tank you have. Then take a 10-12 foot ratchet strap and make an X over it and secure it. The hangar is $12 and can be removed easily. Here is a pic of the hangar.
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