That's more or less what I've been doing, in a slightly less organized fashion. Getting rid of the drafts was a major improvement, too! My GH has a raised floor, so I finally lined it with 2" styrofoam sheets underneath. I have bubble wrap inside, and a solar pool cover outside. And when it gets really cold, I cover the flats (which are on heat mats) with bubble wrap covered with a blanket or towel. This has been an ongoing learning process, and each little addition, as needed, has made a major difference, between 5 to 10 degrees for each layer. Why, yes, I do have an impressive array of thermometers.