Well you are going to want something that circulates the air and not draws outside air in otherwise your smoker is going to be struggling to keep the proper temps....
So you could put a fan like I have in there. You would need to make a metal plate to cover the hole and have a small hole in the plate for your fan spindle to go through. Then the fan would mount to the outside of the smoker. A simple switch could control the fan.
The fan would also improve your smoker's cooking ability by making it convection. The air will be circulated in the cabinet and naturally make it's way up and out the vent due to natural heat rise.
The reason why I chose the fan I am using is because the fan is designed to run for long periods in a extreme heat environment, has just the right rpm for proper circulation, the fan is designed to circulate the air against the cabinet wall, and is a slim design.
Now just to locate the proper fan...
Here is a new one for $45 on flea-bay...
http://cgi.ebay.com/NEW-WHIRLPOOL-OVEN-CONVECTION-FAN-MOTOR-W10167917-/170648825266?pt=LH_DefaultDomain_0&hash=item27bb766db2#ht_879wt_1270