Here's an old farm shop paint trick--
To paint rims without getting paint all over the tire, simply wrap a piece of flat metal flashing into a cylinder that fits JUST outside the bead ring of the tire and tape it together. Lay this cylinder on top of the tire and carefully paint the rim-- the flashing will catch the paint overspray that would normally stick to the sidewall of the tire.
SO, what I'd do, is cut a piece of cardstock into a strip about an inch wide and about 6 inches long (or longer is the circumference of the window is larger) and over lap it into a cylinder, and carefully fit it in the groove around the window, and tape the end to itself to make the cylinder. Now, given that this is SUCH a small target (considerably smaller than a car rim) I'd put masking tape around the outside of the cylinder to keep the overspray from getting to the nosecone. One could also cut a hole in a sandwich bag the size of the window cylinder and then tape the baggie to the cylinder, and install it over the cone. Voila-- instant mask!
Good luck! OL JR