Putting a fan on the shelf is not a good idea since it will vibrate and will be noisy. I use a fan that is hooked on the (upper) shelf. This fan is originally made as aquarium cooler and has special clips for hooking a shelf. I have changed the 12V transformer to a multi voltage transformer, switched to 4.5V so the fan is very quiet.
http://www.shopjebo.com/5/multi_fan_cooling_system_detail.aspx
good luck !
http://www.shopjebo.com/5/multi_fan_cooling_system_detail.aspx
good luck !