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.
You can avoid reality, but you cannot avoid the consequences of avoiding reality – Ayn Rand