I had a squirrel problem a few years ago that was solved in a similar fashion to yours. Last week I heard a scamper not in my attic, but in the space between my first and second floors and have arranged for someone to fine the hole, put the one way "door" in, and then seal the space.
I live close to a few hundred acres of woods and we have lots of wild life. Squirrels are in abundance and while they are a PIA for what you're going through and less offensive things like digging up flower boxes I marvel at their collective athleticism.