I took a lot of your input and came up with a solution that did not require repairing the hole or having to enlarge it!
Here is the hole from removing the vent fan.
Here is the solution.
And one more picture.
I used computer fans (thank you for the suggestion). But since I didn't want to cut a bigger hole and I wanted to keep as much space as possible, I used very small fans. They are 60mm x 60mm x 25mm. I found them at Fry's. They are very quiet and draw very little power. On the other side, they move much less air than their larger cousins.
Tomorrow I will finish the wiring and pick up the vent for the outside from West Marine!