For years, I built a lean-to over my pond covered by swim pool solar blanket, and installed a door using a zipper from Home Depot or Lowes for use on poly in dust containment in houses. Just the cover will keep the temperature up some, but the addition of heat, as in paint bucket heaters, immersion heaters, or other heat sources, the temperature can be maintained much warmer than an exposed exterior pond. I started to try to keep the fish active, and maintained a temperature of 60F for years and then the last couple of years with only one pond, I kept them at 70F. Cold fish sitting on the bottom of the pond all winter tend to come out of winter with belly sores, and I wanted mine to not rest on the bottom that way. It can be cheaper to heat a smaller volume of water in a Q-tank in the basement, but small water volume with high fish loads can be more difficult to maintain water quality.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard