Assuming the window glass is flat (not decorative like some are) then it should be fine. I'm currently using the lid off a fish tank Go to a glass shop and get a bit of 4-5mm thick glass with beveled edges (for safety). I recomend getting it slightly longer than your baseboard to make it easy to lift off the negative(s) and you'll keep your finger prints on the edges away from where you place it on the neg.. Should only be a few dollars.