I had a B&M cooler, mid size (6x9 or so) going from memory. I bought a cheapie Summit fan that pulled the air. I mounted it on the passenger side, up in the front corner blowing down. It sucked cool air from the front grill area, whatever didn't go thru the radiator.
2 tricks to consider:
The puller fan mounted under the cooler kept rocks & debris from hitting the cooler. Works better than pusher fans with the cooler on the bottom.
Tilt the cooler/fan assy slightly away from the vehicle. It can kick up a lot of dust depending on what you drive on. Blowing it away from the car helps keep it clean a little better than blowing it straight down or under the car. Also don't want that heat under there either. A deflector works well too.
Hope that helped.