Polaris xpedition adv
Try this I use it on my coolers:I just put that on mine was a small gap same area as yours I just cranked down the mount a little more, I do need some weather strip at the bottom of tailgate and bed to seal out dust and water.
Thanks Beercan .Try this I use it on my coolers:
Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16" W, 1/4" T, 17 ft L, Black - - Amazon.com