I used a small rod that I shortened to just a few inches wider then the window. My wife sewed me a double sided curtain that rolls up and is secured with velcroe. Works great, I do all my trailer curtains this way.
We made our curtains out of a cloth shower curtain with the metal grommet rings at the top. They hang from two cup hooks at the upper corners of the door and are opened by simply dropping one side and re-attaching to close.