The li jump-starters are designed for brief high-amp loads. You're looking for deep cycle use - exactly the opposite. You say a "boat battery" isn't an option. Why not? Buy a proper deep cycle battery - but put it in a $10 battery box you move around on a $25 light-duty hand truck (the $25 kind that fold up). My $25 folding hand truck from Sam's Club is used to move around the Group 27 battery in a box I have for my pond boat, works fine, and is a lot cheaper solution.