Just installed mine without trimming the grip, fit's like a glove that way but I may still trim it some. Mine came with 2 gaskets, one for smaller grips that was thicker and I used that one. Appears to hold the grip very tight. If it doesn't hold, I'm thinking trimming the grip will be necessary as it does get thicker as it approaches the switch-side and it may need that thicker area to grab and hold. I did turn the set-screws in quite a bit so that helped also.
I'll post back when I have more to report but an easy fix would be either trimming or find some longer set screws.