Hi, I have had budgie for many years when I was younger. All my breeding boxes never had any material in them and I never lost any eggs or babies. BUT recently I have decided to breed again with my children, and as a start we made our own boxes as a family project and to save money. These boxes do not have the concave in them and not sure if I now need to put material in our how to make the concave. The boxes are deep with a 6cm round opening and has a hinged roof for cleaning and checking. I'm pretty sure I need the concave, but can I use something else?