Help needed for a bird box

Has anyone got advice for me re slugs repeatedly getting intro  our bird box?  We had the beginnings of a nest and then all activity ceased.  We didn’t want to disturb the box in case there were late nesters needing it but when I came to clear it out it was full of slugs.  This is the 2nd time this has happened.  Any advice?  I have now put the metal bands I use on my plant pots around some of the rim but am not sure if this is going to work….

Helen Griggs