I use a 20 gallon plastic tub buried up to the rim. The bottom is about 2 feet below ground level. I made a 4'X4' cover that's insulated with 2 inches of foam. It has a door to get to the tank. The closest it comes to freezing is a 1/8" skin of ice. I don't use an aerator because pumping 0 degree bubbles into it chilled the water enough to freeze.
When I used to dip smelt in the spring I was able to keep them alive until late July in the same setup.