OP said little fans, so unlikely he is talking about a Fantastic or Maxair fan, he means the cheapo vents with the small fan in the corner.
I would just get the $12 computer fan, it is in a frame already, and will run off of 12 volts. It will move air, but not sure if more or less than what you have. You are not needed a huge amt of air moved or you would already be looking at the Fantastic fans.
A friend used 4 of these computer fans together which took up the entire 14x14 opening and it moved more air.
These are super quiet too.