1/2 x 1/2 would probably do it, but that is probably something that you don't want to have to do or buy, especially after just building it.
I throw mothballs all around my pen and buildings, plus I try to keep any mice dead through various methods.
Now that they know where the easy food is at, you may have to kill or catch it/them..............or go with the 1/2 x 1/2 wire?
Good Luck