Or, just get a shoebox! If you have a door (where it can and WILL escape), then the top of a shoebox will do as its normal lid. Yes, many shoeboxes do have ventilation, or you could put VERY small holes if it doesn't, however, they can breathe just fine in their natural mud & grass nest piled under multiple siblings, sometimes with their mama on top. I wouldn't be worried about ventilation, I'd be more concerned that any ventilation you provide exposes the baby bird to temperatures too cold for it. I suggest a Swiffer (brand of feather dusters). It looks & feels just like feathers, is just the right size as a "blanket". If it doesn't like it, it will move (e.g., to poop).
