I would extend it to the width of the bottom cabinet or 24"; whichever is easier. The problem of making it the width of the nest boxes is if they perch on the edge of the tray, the poop will fall to the cabinet below. I used 2X4's for the perch because we get below freezing and the width helps keep their toes insulated under their feathers but, if you're in Texas, you probably don't need to worry about that lol.