Well from what I hear the ammonia in windex and other window cleaners will dull the finish on a plastic display. I’m not sure what size screen or resolution you have but on the newer eagle fish finders they are puting a 640 x 480 resolution on a 5 inch screen so they naturally put the clearest plastic they can.
I have seen some posts on the internet when I google the subject with a variety of responses but I hadn’t seen anything that seemed simple or cheap other than water and a soft cloth.
Well maybe others might have some better suggestions. I didn’t see anythng on the eagle website unfortunately.