Several people have replied to cover with a hand towel. One towel covering the brooder will help hold in some heat. However, I would recommend two or three layers of towels over the top of the brooder. Just like dressing in layers in the winter. And it would even be better if they draped all the way over the sides to the floor, helping insulate the brooder as well. A box inside another box might help hold in the heat.
Good luck.
Good luck.