I took a gallon water jug, stuck it in the freezer, then put the frozen jug in the foot bath. There is still room for one chicken to get both feet in, and they all enjoy laying around the foot bath. And the ice lasted for hours, then I used the cool water to rinse refill the foot baths (because, ew, they get dirty fast).

What with the ice cubes, ice packs for the cooler AC, and jugs, we had to start up the large freezer in the garage.