Are you talking about the R9 ?
Eitherway, about 10 years a company made some add on doo dads that did not fly and they disappeared from the market.
Bassically, they angled plastic pieces perpendicular to a flat piece (attatched) which the flat piece fit under the grip panel and through the grip screw hole as well. Then sticking out from the grip panel would be a angled thumb rest.
I'm making it sound more complicated than it is.
Just think of holding a 1/2 inch wide piece of 1/16-1/8 thick steel 2-3 inches long in left hand gripping it. Now take your right hand and bend it 90 degrees and whatever cant you like.
Drill a hole large enough for the grip screw to go through and there you have it ---
Except - you may need a longer grip screw.
Off to the local Nut, Bolt and Screw House.
Good Luck