I also have a gap at back of loft and have three foot louvers running the full lenght of the front of the loft . no way will i ever close off the airflow as fresh air costs nothing and stale air causing health problems (to pigeons and fanciers) will cost you a lot .
I remember a long time ago an article written by "Old Hand" in one of the pigeon papers that the best loft consisted of three wire walls and a solid back and roof
plenty of free fresh air ,bloom blown away no PFL no draughs healthy pigeons .