So there I stood, looking into an open hole thinking "Insulate box, insulate box... hmmmm." And inspiration struck!
When Robin and Summer left two days ago, they left behind an old cooler whose lid had warped. It was a matter of a few minutes work with the chainsaw to slice the bottom off of it and fit it over the water cut-off handles. I then seated the lid as best I could and covered the whole thing with dirt. The dirt will keep the lid in place. The cooler will keep the pipes from freezing.
And I didn't have to waste wood.
This is a perfect example, I think, of reusing a piece of potentially toxic trash. What ideas do you have?