I use the 10 gal. Rubbermaid / Home Depot cylindrical orange cooler. Most of my beers fall into the range specified, and it works just fine. I will lose maybe 2-3 degrees in a 60 min mash, and this makes absolutely no difference, as far as I can tell. The lids to these coolers are uninsulated, so I injected the lid with some of that "Great Stuff" insulation in a spray can, and that improved matters greatly. I also cover the cooler/MLT with a furniture pad while mashing.