Rework the rectangle. Consider creating a brochure that’s square or round. Printers often have pre-made dies in several sizes to round corners or cut a custom shape that can add character to your design.
baseboard trim on rounded corners - Google Search
Installing quarter round on round corners, here is the finished corner after installing the quarter round and chalking it.
How to wrap bookcloth around round corners
Baseboards Around Rounded Corners - Carpentry - DIY Chatroom Home Improvement Forum
Installing quarter round on round corners,this shows how to cut the quarter round at 22.5 degree so the quarter round can be installed on a rounded corner.
How to get around those pesky rounded corners! (Website doesn't work, but you can figure it out from the picture.)
contemporry corner fireplaces | Modern Design Ideas For Round Corner Fireplaces
Hey, I found this really awesome Etsy listing at https://www.etsy.com/listing/174152231/200-business-cards-frosted-plastic-stock
brick corner fireplaces with mantle | Brick Corner Fireplace Accent Walls | Corner Fireplaces Design Ideas ...