I have used a heavy furniture moving pad. I have a Brinkmann Smoke N Pit. I open it all the way and fold it in half on the long side and wrap under the bottom and up through the shelf area and clip it on with bulldog clips. Then pull the other end up and around the body of the smoke chamber and lay it over the top. It looks completely ghetto but it is very helpful for cold days. A lot of times it is raining and cold so I have to "help" the bbq out. As long as mine doesn't get wet the pad works fine.
Wool blanket from an Army surplus store works good too and they are cheap. I just folded mine in half then cut a hole for the chimney. Wool has a very high burn point too, if I recall it is over 400 deg.