Hi, I hope you can get more ideas, but one option is to get a piece of plywood and cut it to the size of the seat and mount it by using sheet metal screws around the perimeter so to take advantage of the strength of the bends near the edges of the sheet metal, then mount the trolling motor mount to the plywood directly. I believe mounting directly to the sheet metal with it being as thin as it is could cause flexing and maybe later have to be repaired. The foam is flotation for safety, but I have read where some remove the foam. I have a 14 ft. Crestliner jon boat I purchased new and the dealer told me that the weight ratings on the tag on the boat was misleading. He said that with the flotation foam in place that the boat full of water would still support the weight that your boat was rated for. I think your polar kraft is rated for 325 lbs., so it would still support that much weight full of water. Good luck and enjoy your new boat.
Jim