What to block a hole with to stop mice getting in?
When sealing a hole around an electrical entry cable, the best product to use is Plasduct. It is a putty like material, gray in color. You will find it in most electrical departments at the box stores or your local hardware store.  Use it just like modeling clay, work it a bit to make it pliable, then stuff it into the hole around the cable as deeply as possible. I have never seen rodents or insects get through this stuff.