This may be one of the best training aids I’ve ever used. It’s a $10 dog ball from Amazon (the 5.5 inch (JW Pet Hol-ee Roller Dog Toy), a 2” x 12” Velcro strap, and I blew up a balloon inside the ball to get the level of firmness I wanted. The ball by itself was too soft. The goal is to keep the ball firmly between both arms through the entire swing.