I just put them on 12V 1A adapter parallel, one on each side, you can find on aliexpress or ebay the ones with speed control for around 5$ but no need since you want them on max power...2x 2 inch M4 screws up and down on each vent to give them room to breathe...those on the pics are 120mm, i also used 3x 140mm vents on each side for my 2.75x2.75 which are a bit better in my opinion
4x 140mm on each side would be perfect for 3x3 but not sure it would work in a 4x4 unless it's a really powerfull pc fan, that seems like just a bit too much space from one side to the other