Domed, I presume?
Before you do anything, put it in the cooler and see how it sits. You never know, I doubt cooler liners are dead flat either, so you might have a good fit already.
In any case, I would spend the time to get the FB to make contact around its full circumference with the cooler liner. I have a 12" domed FB in my 10g Rubbermaid and it's made from some pretty stiff punched stainless steel. I doubt even a 20-something pound grain bill is bending it much if at all (hard to say under all that mush
Anyway, it took a little time to get it to lay tight to the bottom, but avoiding grain clogs in pumps, valves and and chillers makes any preemptive activity worth the effort...