Instead of using tennis balls you could try a handball. no fuzz and you would be able to cut along the seam to keep the cut straight. Everything is looking FANTASTIC!!!! Great job man.
Unfortunately the handball was too wide as well, I believe your referring to the blue bouncy ones right Dirt? If so I also had one of those lying around my house and thought the same lol, it would have been perfect to but just a smidge to big ...