I've never ever had good results with any window masks. Never fits 100%, and you get bleed.
What to do: Draw lines with an overhead projector pen on the outside. Use a brush (don't need to airbrush) and put on 2x thick coats of Bob Dively Liquid Mask. Cut along the pen line with a sharp Excel knife. Peel of the excess. Paint. >> Perfect
Not to forget you can then also make your own "custom" windows!