Welcome to the forum and sorry to hear of your injuries, I hope "slinging therapy" works for you.  I have played around with multiple shots too.  I found that a smooth shallow cupped pocket worked best for them.  With a single shot spin, and therefore a pocket with a bit of roughness,  is very handy but with multiple the smoother the better.  I used to use a reversable pocket, corse canvas on one side and leather cloth, pvc side outernost, on the other.  With modern materials maybe one could cut up a teflon baking sheet and use that, no idea if it would be tough or flexable enough to hold up but if it did it's be excellent for multiple shots, very slippery.  I also played with slinging shingle wet or dry.  It seemed to make a slight differance, wet grouped a little tighter,  but not much and was a lot slower..  Makes a great looking spray cloud on leaving the sling though.