Here's what I did w/ mine... killed two birds w/one stone...found a 120 qt cooler to fit hole for fish-box, then custom cut 3/4" Starboard to cover remaining opening...as MJ said, you'll have to put support-cleats on the inside of the hatch for this to work (also made of Starboard and screwed to sidewall)
Good luck w/ yours and post some pics of what you do...we're pic-hounds around here ;D ...
...keep in mind, MJ's and mine are early '70's hulls ... yours will be configured diferently, but the same kind of thing will work for yours ... ;) ...