Hey, not exactly sure if you're referring to an inline fan or a clip fan for circulation.
Either way, for the cloudlab 642, a 4 inch inline fan would be perfect as it won't take up too much space. I would recommend the cloudine T4 from AC Infinity.
As for a clip fan, you'll do just fine with one 6 inch clip fan for circulation. I would suggest the cloudray A6 from AC Infinity.
Hope this helps!