The computer fan you posted a link to helps alot. The fan in the rear will depend on many factors.
Where your fridge is located...in a slide?
The venting behind your fridge.
Do you have an upper access above the coils?
The ambient temps you usually camp in. Under 90* is usually better.
If your venting is good you may not need one. If it's in a slide it may already have one.
If you have access to the area above the coils you can put a multiple set of fans to create a really good draft.
try one of the inside the fridge fans first and see what kind of results you get. Add fans behind after that.