Use a piece of astro-turf in each box as a nest pad. If they get dirty just shake them out or blast them with the garden hose.
These are the type that are used in commercial nest boxes. They are basically astro-turf, but are perforated for dust to go through.
These are the type that are used in commercial nest boxes. They are basically astro-turf, but are perforated for dust to go through.