I recently installed a Caframo Sirocco in my camper. It’s not quite clip-on but very quiet and efficient. There are two models, I got the cheaper as the direct drive was out of stock. They are used on a lot of sailboats and come highly recommended. Check them out at the Caframo website or on Amazon, I connected mine to a 12 volt lighter plug and mounted the bracket near by or you wire them into 12v direct.
Just know that most of the noise from a fan is from moving the air. Smaller fans typically need more speed to make up for the smaller fan blades, and therefore they can make more noise. Cheaper fans will also use fan blades at a much flatter angle, so they can get away with a less powerful motor. Lots of tradeoffs.
SMB-less as of 02/04/2012. Our savings account is richer, but our adventures are poorer.