You are better off just getting more radiators because the radiators you already have will always keep your liquid temperatures above ambient so there is no point in having the fridge.
Also:
- A mini fridge has only around 50-100W of cooling power which is similar to a 120mm radiator.
- The compressor in the fridge will die quickly as it is not meant to handle a constant heat load.
The mini fridge is basically just a big, loud 120mm radiator with a limited lifespan.