How do you cover a 40 inch round window to prevent sun damage?

You can do reflective film, like on car windows, which allows you to see out but will cut the sun and ability to see inside.

Static cling film will also work. Rabbitgoo is the brand I used and love it, no sticky residue.

You could cut a cardboard circle or a styrofoam circle to fit. You could add some decorative fabric, contac paper or wallpaper to make it look better.

Contact paper will most likely soften, come undone, or get gunky in the hot sun.

window film you may have to piece a couple of sheets side by side to get complete coverage This is 1 company https://www.suntrol.com/sun-protection but Loews and Home Depot also sell window film

You could take you sun problem to a local company, you could also check out companies like Blinds.com. They might have suggestions to remedy this.

You need heat reflective window film, UV screening window film it comes in 4 colors - platinum, blue, smoke, bronze. Amazon has a really good selection in various sizes. Easy to apply. https://www.amazon.com/s?k=heat+reflective+window+film&ref=nb_sb_noss_1

If you go with window film don't order it online. This is a product you want to physically see and touch. There are several rip off companies out there that sell what basically amounts to black see thru plastic. Call a business in your area and see what brands they recommend.

Hello Barbara, have you thought of placing something like a shade or blind over it? I will help reduce some of the sun from getting in fading your interior, I found this blind that may work for you

https://www.amazon.ca/Original-Light-Filtering-Fabric-Shade/dp/B00KWKY7IY/ref=asc_df_B00KWKY7IY/?tag=googleshopc0c-20&linkCode=df0&hvadid=292959013912&hvpos=1o4&hvnetw=g&hvrand=8457216995994116998&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9001388&hvtargid=pla-312007656618&psc=1

Circular windows can be covered with cellular shades or wood shutters. A cellular shade of this style will be inoperable, but can be made with fabrics to match other shades in your home. Browse the Bali fabric options online OR try the Shutter store. https://www.theshutterstore.com/shop/styles/special-shapes/circle-3

Window film comes in several types of heat and light reflection tints like bronze, smoke, blue and platinum..... Amazon has a great selection..... https://www.amazon.com/STATINT-Rejection-Non-Adhesive-Reflective-Brown-Silver/dp/B07W6FYK1H/ref=sr_1_19_sspa?crid=2Z4P3K3QQHXLP&dchild=1&keywords=heat+reflective+window+film&qid=1607053660&sprefix=window+film+and+heat+refl%2Caps%2C240&sr=8-19-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExSTJORlpSQ0U0TVNDJmVuY3J5cHRlZElkPUEwMzIzMTc2MlpQU0lUQkNFWk1NTyZlbmNyeXB0ZWRBZElkPUEwMDA2ODUwMUFEQlRZQ1BTOU1TViZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=