I have that stock on my 870 with a red dot sight. I made a "riser" by using foam like you find in a hard gun case and some that is sometimes used in shipping.
Cut it into several strips that are the length you want and about 2.5-3" wide. You can stack them on top of each other or tape one at a time to the top of the stock with electrical tape. You'll need to use several layers and make it taller/ higher than you actually want it, because the tape will compress it as you wrap it around the stock. I finished mine with a roll of the "cloth" type camouflage tape. It doesn't look bad and it is effective.
I tried the Beartooth kit first, and it didn't work on this stock. Method above works on the same principle, just a bit more crude...