Several times while visiting large pancake and other breakfast restaurants, simply by asking I have gotten large stacks of flat gray cardboard egg trays that hold 30 eggs each in a 5x6 compartment layout. These are stackable, and they can be cut for use in the incubators. They have a lot of uses for chicken owners. The restaurants are glad to give them away because they cannot be used again; it saves them the trouble of disposing of them. All you have to do is ask the restaurant manager for them. While you're talking to the manager ask him to save a days worth of egg shells. Take them home, put them in a hot oven until they're all really hot, take them out, put them in a paper bag and smash them into small pieces for your hens.