I put a piece of blue foam board EPS on top of the inner cover. The foam had a hole cut to match the cover. Then on top of that went a medium I'd cut in half. Then on top of that medium went another piece of foam, then the outer cover. Then I put foam around the whole hive (including this top box) and held it on by wrapping it in roof felt. I know, I know, totally overboard and nuts...but I think on those uber cold nights, even though there weren't that many this year, the girls didn't have to work as hard to keep warm. I'll do it all again next year. The upper box is now where I have a big chunk of fondant which is being eaten rapidly on some hives, only picked at on others. Another reason I did this was even though the notch in the inner cover was providing some ventilation, I wanted that inner cover hole to have a nice outlet for moist air to go, and if it then condensed on the foam that was right under the top cover, very little would drip back on the bees, it'd just drip back down into that top area. One more detail...I did all this in early November and we still had flying days. I didn't want the bees doing much in that top box, so I used a little piece of hardware cloth to cover the inner cover hole, and put it between the inner cover and the foam. That kept the bees out of the top area until I was ready for them to be there a couple weeks ago, but allowed that moist air to get up there all winter.