Glad you got some use out of this shamrock. Ed, I have no clue why I never got notified of your response, or any after it for that matter... Anyway, I like your idea about the giant Koozie, but I have found that it wasn't necessary with small groups. The beer spends enough time in the ice at the bottom of the cooler to be plenty cold.
I do think it would help the ice last a little longer, since the keg was still pretty cold at the top due to the thermal transfer, but I didn't use but a couple bags over a 4 day weekend.