I have a round porthole window in my door and two oval portholes at the front of the house (on either side of the cab). It came with snap on covers, but I hated them. So, I got some window film (
https://www.wallpaperforwindows.com/pc/Window-Film-All-Designs-c61.htm) and used that.
I got the opaque so you can't see inside - but it still lets some light inside.
The ovals are a green film:
And the round is an oval with a cutout (so I can see out):
They stick very nicely, yet you can remove them if you change your mind. And there are so many colors, patterns, and styles to choose from.
