When we first got the boat my Mum made us lovely porthole stoppers. They were made of foam discs covered in material. As time went on I discovered the foam was'nt quite rigid enough to hold the stoppers in place. Eventually after wondering what I could use as a rigid frame I spotted Hoola Hoops in the shop where I work. You know, the ones you put around your waste, and Hoola!.
I stripped the bright pink plastic off to reveal white tubing. Perfect. I then cut to the size I wanted, and used original material to cover hoop, gather into the middle and add matching button. I also put blackout material inside the hoop so it would block out early morning sunlight. I sewed a little loop on to the stopper to make it easy to remove from the porthole. Because the stopper is now rigid, you can have window open behind it to cut down on condensation. Job done!