Just got the Fogbuster and it was initially a pain to fill up. What I ended up doing was upgrading Cairo, which increased its layovers from 20 to 40, making it a little easier to make use of its larger capacity. Cairo is the mid point for my flights, so it gets the most traffic. My flight empire now stretches from Los Angeles to Sydney.
Doing the same, but oh wow at 102k required for upgrade to L2.
I also do aim to have a few large 'hubs' for the C3 planes to fly back and forth from including Cairo.