Somebody suggested strawberry containers to me when I was wondering this. I haven't tried it yet, but it should work. The strawberry baskets in particular are open aired. If you use the clear plastic type of containers, I would remove the tops to ensure proper airflow and humidity.They might work fine, but I would be a bit concerned about air flow, depending on the type of basket. Pay attention to the height of the basket and the heating element on the incubator. Don’t want it melting, or a chick trying to climb over and getting burned.
In the Article section here, someone made some neat dividers from the plastic crafting grid stuff. I can’t remember the name of it.
Others have used mesh laundry bags successfully.