From experience it is best to have 16" deep. From your measurements, is that vertical nestbox or horizontal one 2 feet long? If it is 2 feet long, 8" high, 8" deep, then it might not be enough because nest bowls are like 8 inches. They will be on the edge. From experience, pigeons like to mate inside the nest box as well and 8" high makes that impossible. I have 12" high and the birds have difficulty mounting each other. Their heads touch the roof. I have the same measurements as moonshadow13.