overheating computer

I recently bought a computer to put in my new desk, to my dismay it keeps overheating, air gets blown out the back, hits the back section of my desk, which has a 9cm gap down the bottom, then pushed back in through the side.

I have looked around and the best I can find is a fan to attach to the expansion port on the back of my computer. So I was wondering if anybody knew of a product that would allow me to attach a fan to one or two 5 1/4″ bays, or any other means of cooling (that isn’t overly expensive).