Try using heavy duty aluminium foil. If you fold and glue 3 to 4 layers together. Then glue to a thin sheet of plywood that extends slightly beyond the edges of the birdhouse. Tack the plywood to the top of the birdhouse. Aluminium foil freaks my cat out. She doesn't even try to get on the patio table anymore after several times of skidding across the foil. My neighbor used it on his tables and counters tops and said it worked on all of his cats.