Celtics should hire, if they don't already have one, a really good shooting coach. Use of a paddle as a prop seems kind of dumb in my opinion.
When I was a kid, I would tape a pencil to my shooting hand palm, to make sure the ball stayed in my fingers (something I was taught at Sam Jones basketball camp, LOL) but at least that did not interfere with how the ball felt in my hands. Using a paddle doesn't make sense. Your off hand should provide guidance, it just shouldn't be pushing the ball, especially with the thumb. The off hand should be end up perpendicular to the ground is all.