One is to get some magnets – you can find some peel and stick flexible magnets that are for example designed to convert business cards into refrigerator magnets. use a pair – and place one on either side of the screen (nylon or aluminum) and they will hold to each other through the screen via their own magnetic power. Attach the (revealed by removing the waxed paper on one of them)desired design or sign to the sticky adhesive coated side.

Another easy to locate item are placards or tiny 3-D plastic painted figures which have been cut in two, and have a recessed (placed directional N-S pole magnets) magnet inside – you place one side of the figure on the outside of the screen and the other side on the inside – again the magnetic power of the magnets holds the item in place.

The other idea is to use screen paint and paint a design in bold contrast color upon the screen (in the early part of the last century it was very fashionable to have mural images painted on window screens!).