Blowholes are good. I have one in my case. just make sure you line up the hole far enough in front of your PSU so you have room to mount the fan.
A fan centered over the CPU in the side window is a good idea, blowing external air into/onto the HSF. If your HSF sucks from the side, put the fan in front of it and put a 90 degree duct on it to direct it into the HSF.