I'd recommend www.levergunleather.com For about $70 you can have a plain butt cover that will solve your problem and dress up you rifle as well. I doubt that anyone makes a slip on pad that will fit your rifle. Lever does beautiful work, but it takes a few months to get it. You have to trace the outline of your stock in order to get a perfect fit. Maybe someone else has a better idea. This is what he did for me on my old 94 Winchester 38-55.
Man, I really like the idea and the look of that leather butt-cover! There are a couple of leather workers around here I'm going to check with but will be using the rubber non-slip piece for now. Thanks a bunch!
"History teaches that war begins when nations believe that the price of aggression is cheap." - Ronald Reagan
I have never used a leather butt cover. I have used an elastic butt cover/ammo holder. I didn't remove it for quite a while after I installed it. When I did remove it, there were a bunch of sticks and dirt under the cover. I would take the cover off after every few hunting trips to make sure there is no nasty stuff under there. I was lucky! Old English scratch remover got all the scratches off my gun.
Speaking of luck. I create my own luck when removing scratches. When I get a new gun with a wood stock the first thing I do is wipe the stock down with Old English scratch remover for light colored wood. Since the oil has already pigmented the entire stock when I do scratch the stock one or two light applications covers any scratches on my wood stocks. I have several guns that are over 40 years old. The wood stocks all look like new due to Old English scratch remover.
Yesterday my wife shot her Golden Boy for the first time and experienced the same problem with a slippery butt plate. Answer: Since there are not slip-on rubber pads available that will not make the stock longer my answer is to find some rubberized drawer liner and sew a chunk of it on a light weight jacket on the correct shoulder ...that should stop the slip.