Use a neoprene washer then a SS fender washer over it. You can get the neoprene washer at Home Depot. Should seal it up real nice.
I may be wrong (as I am often
) I think the cooler is made out of ABS plastic. Check into it. If it is ABS an easy way to make a quick and perm repair of ABS is to use some ABS pellets or if you do not have any take a few Lego bricks (they are made out of ABS)and dissolve it in some MEK (methyl ethyl ketone). Use just enough MEK to turn it into a thick sludge then apply it to the cracked area and let cure. It will melt right into and bond with the existing plastic for a solid perm fix.