I usually stick an 80mm fan, in the empty bays with double sided tape, blowing in - with a backside fan blowing out. It's good for a couple less degrees as it blows right on my cpu heatsink.
Well, I had a junk power supply so I cut off the backside as it fits perfectly into the bays. Now I have a mount for fan and a switch for it or whatever. I have to put some screws in though to cover the holes.