My son needed some thing to help him move in the crease in the butterfly, especially since he's a little undersized....here's what I built:.
The plates are made from the same plastic that roller fly uses but without the ball bearings. I cut them to fit his pads with my scroll saw and beveled the edges with a belt sander to keep from catching on sport court, then attached them with industrial Velcro to keep from damaging the pad.
I'm going to make a set for myself also as he slides as well with these as I do with roller fly.