For the second level I cut out compartments from a 1/4" piece of foamcore and mounted on a piece of posterboard, both glued together and cut to fit the case. This level accomodates the tiny flyers, tiny tanks, escort ships, cards and templates, along with the British submersibles I've added to my fleet. Note: not all of my models have been painted.
Still working on the top level which is another piece of foam, only 1" thick, which accomodates the relatively low profile airships, carrier, and gunships. And I still have room to add new ships as they come out of drydock!