The 425/450 are going to be about 1 mpg depending on how heavy, my 2017 with 450 goes from .9 full load to 1.2 as it empties out. They are 900 lbs a piece
Boat outfitters has what you are looking for, the smaller version is what you want, overhangs the opening by an inch on sides. Changed mine last year and put in a retractable arm