Ummmm - I was going to say - isn't there some way of sort of putting a
small, ummm electric fence in there. Just turn it on at night????

Then I went and queried it with DH (he is an electronics engineer),
and he totally confused me with the pulse rate of electric fences, and
what it would do to them if they were wearing a pacemaker.

He suggested a lemon tree. Not much help, I am afraid.