For Art1Top, you need to attach the fans first to the “Art1FanHolder” part, and then attach those to “Art1Top”. The fans are 40x40mm.
I just realized that for BaseBoxBody the holes are too large for a 40×40 fan. Not sure what the deal is with that. (Still going through the process of building the robot myself). I might design and print an adapter, since I bought all the fans already.