Create an easy insulated water garden with a sunken container. Rather than placing a container on top of the soil where it's susceptible to temperature fluctuations, plant the pot in the ground so it's surrounded with soil. The minipond also looks particularly attractive at the edge of a path or when surrounded by other plants.
Using the pot as a guide, dig a hole slightly larger and deeper than the pot, so the rim sits flush with the surface of the ground. Level the bottom of the hole. Add a layer of sand or gravel if necessary.
Set the container in the hole and fill in around it with sand, gravel, or excavated soil to make sure the pot is secure in the hole. (Find a use elsewhere in the garden for the unused excavated soil.)
Almost anything can be used for a water garden. Turn an old or new metal trough—horse, cattle, or pit (available at antiques stores or at animal supply stores)—into a water feature simply by sinking it in the ground; follow the same directions as you would for a smaller container. Add one or two medium-size rocks to connect it visually to the surrounding garden. Camouflage the rim with a covering of rocks and flat stones, or leave it in view, accented with plantings. Put a fountain near one end.