Honestly, I think SSB is the king for Wide Box Squats. IT forces you to stay upright as you drive off the box, which is exactly what you want when you drive off the floor. Plus it helps work around the shoulder issue. I'd def pick one up if I were you.
Maybe throw in Dead Stop Leg Presses. Kind of mimics quad drive off the floor.
Sounds good........I will invest in one maybe when my training partner gets back next month we can go halfs.
Thanks for the help I will post a video soon of some sumo speed pulls and work up to 500+ to so you can see what I look like at 90%.