Originally Posted by mruffruff
I should add that the weave on hardware cloth is bigger than ordinary screening, so it won't keep bugs out. You might want to have both. If you can, attach it to the window frame with a strip of wood over itfor stability.
I have part of an old metal crate propped between my window and screen to keep the dogs inside while the window is open. It looks like a refrigerator shelf, but it does the job. The size is important; has to be wide enough to touch both sides of the wood but narrow enough to put it in. It only covers the bottom half, but the storm window covers the top anyway.