Fred mentioned above how you need a good 3 inch lip to hold the nesting material in the boxes. I use hay, it has seed so don't compost it but as I'm constantly redoing sections of lawn I tend to have partial bales in the shed so it's readily available. I did use straw for the purpose of throwing it in the compost bin when used and no fear of seeds to weed the garden later. Basically I've come to the conclusion very little hay or straw is used for nesting. If it gets dirty I flip it over and believe it or not I can usually flip several times and still have clean eggs so there is very little material used.
