It's not the size that's adjustable.
I make individual sizes for my Dixie Rainbow, Barred Rock and Serama.
It's the ventilation, temperature and privacy that is adjustable.
Once a bird gets locked into setting on her eggs, I can remove the covering and let her get some air in the heat of the day during the Summertime.
You can use an old quilt or sheet or tarp and some hot glue to fashion a cover, depending on the temperature requirements.
You can build a custom cabinet if desired and stack them accordingly.
At 100 degrees over a 3 week period, shavings can absorb dampness and breed germs.
These boxes are very easy to spray clean and then spray with alcohol for a sterilized fresh start.
