I like to use the 4L (1.1ish gallon) fuel tanks. They don't sit any lower, but if larger is what you want, they are great tanks. The main reason I like them is that they don't mount with studs, and instead use bolt through clamps at both ends.
Example of one. https://www.amazon.com/Tank-Motoriz...3&psc=1&mcid=0bd7b76c0c5b38c192c5797fc5eb6b5d
Example of one. https://www.amazon.com/Tank-Motoriz...3&psc=1&mcid=0bd7b76c0c5b38c192c5797fc5eb6b5d
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