The only bag I know of that is specifically marketed as a cooler is the Stone Cold Bicycle Cooler Bag, but I'm not sure if that's quite as big as you are looking for. I claims to be able to hold a six pack. But it is cheap enough ($20) that you could get 2 or 3 of them (they show it being able to attach to either the rear rack or the handlebars.)

Other than that, some rack trunk bags are insulated (mostly so they stand up) and those might serve your needs. Both the Inertia Designs and Jandd should hold about 8 cans or so.

I do like the Wald basket idea, though. Wald does make a insulated bag that fits a number of their baskets (the 14.5x9.5x9 sized ones) and it ends up being about 50% bigger then the trunk bags listed above.