I had a boat that had very clever porthole covers
Fabric to match the drapes was stretched over a hard backing the shape of the portholes--wide enough to cover the entire "window frames" of the ports. It was padded, and then a border added to finish it off. 4 canvas snaps attached it to the frame around the porthole--two at the top, two on the bottom. Snap on when you want to cover 'em, take 'em off and stick 'em in a drawer when you don't.