I think that will do fine for a week or two, if you wrap some cardboard around the outside you'll take care of the draft problem. Good luck with your chicks.
I think the suggestions about putting something along the sides were also to help hold the heat in. I have mine in a platic bin with a window on the side and still have to cover half of the top to keep them warm enough. Otherwise, all the heat from your bulb is just going to dissipate.