I heard that some guys use glass marbles/ beads and fill their jugs with it as the fuel level drops. Otherwise, shouldn't one use an air drier to suck air into the jug as you pump fuel out (air displacing fuel)? These air driers are installed on large transformers and consist of cobalt crystals. The crystals absorb the moisture in the air and turns from blue to pink over time. To regenerate it you just need to dry them in the sun/ low heat in the oven.