Hi gaz the court looks sound mate. But I would recommend Turning your nest box upside down as the under side of the ferrets won't fair well scraping over the plastics lip a few dozen times a day. Cheers king.
I did think about that. I smoothed it off with a file so it wasn't sharp and then wondered about doing something else.
Then I thought, they are tough as old boots working ferrets, scraping over roots and rocks and getting kicked by rabbits and whatever else they come across.
I might turn the box upside down though for peace of mind.
Or get some rubber boot seal from a scrap car gaz. A