Turbo - I broke my neck a decade ago and get all sorts of fun nerve bullshittery from it ( including it turning off my left arm for 6 months in 2012) ..... Donnie, as well as a good friend who is an ortho suggested using a shotput ( or bocce ball) .... tennis balls are too small and soft. Lacrosse balls work well for my 130lb wife, but even she needs to use a bocce or softball to get her traps, lats, and spinal erectors. If you can make it to any of his seminars GO! He's coming to Long Island soon and will be in Chicago in May..... look him up and ask if he's coming to your area. The $50 or so for the seminar will pay off several times.
BI - I used his prototype when I was down there last summer .... I can normally pound out sets of laterals, rear delt flies, and front raises with 50+ lb dumbbells.... the 35lb cannon balls almost killed me.... I don't understand why they are so nasty ( I was too busy asking about the X-Wife to ask about them) ...... they were definitely fun to play with. Honestly, though, for the money I'd start with buying a fat pad and bolting it to a bench in your gym. That thing is doing wonders for my shoulders.