Fridge for fermentation build

What is a good size fridge to use in a fermentation chamber? I'm thinking of building one out of wood with foam insulation to hold 3 buckets.

I can get a 3.4 CU FT one for $30. To keep it at around 68, I figure this one will do really well.

http://providence.craigslist.org/app/2592549173.html

Or I can get a 4.5 CU FT for $50

http://providence.craigslist.org/app/2593175748.html

Or go even smaller with for $15

http://providence.craigslist.org/fuo/2593322253.html

 

You should run the numbers first to figure out how many BTUs you will need for the chamber.

BTU/hr = (Sq. ft. of interior)*(delta T)*(1/R-value)

Then you can compare that to the different cooling capacities of mini-fridges and figure out which one would work best.