What kind of gap did you leave between the box entrance and the guard? They need 3″ to be able to easily negotiate a guard. Some females don’t readily accept any guard so it’s always a good idea to watch for that acceptance and immediately remove it if there is trouble. I understand wanting to protect them from HOWR. Been there for years.
Sorry, but you can’t incubate the egg. The only solution would be to find a nest with eggs that haven’t yet undergone incubation. That’s pretty difficult.
Gin
Atlanta, GA