I like the plan... How about a layer of foam rubber (like a gasket) wrapped in visqueen with the visqueen run wild by a foot or so. Then a clamp over all that. The idea being that you can tape/seal the visqueen to the cooler to seal.
I'll be curious how it works out, because I've got one of those to do too. I'm probably going to be cutting a poly drum in half, but the sealing issue is the same.
Other option I think is to partially submerge and rotate after one side rots off. Don't like that option near as much.
