I just mounted 2 fans inside my keezer. I can post pictures later tonight if I remember.
Using sheet metal screws to grab onto the thin plastic of the underbelly of the keezer lid, I mounted one fan pulling air up on the right side, and the other pushing air down on the left side, to get a circulating type of effect inside the chest. The underside of the keezer lid is ribbed, thus there is some airspace on the backside of the "push" fan to allow it to flow. I still had to offset it from the lid underbelly with some spacers, though, because it wasn't pushing much air.
I used an old 12v DC transformer rated for 500mA. I think it used to be for charging a battery pack for some obsolete piece of electronics. An 80mm computer fan usually draws about 150mA, so I felt comfortable hooking up two of them to this particular transformer.
If all you can find is a 9v transformer, that will work too, just push slightly less air as the fan won't spin as fast. Below 7v you may not get it to power up, though.