The best and easiest fix is to get some of the product used on steps to make them non slip. It's like very coarse sand paper and have adhesive backing. Just cut out what you need to fit the stock end of the adjuster, make any needed holes or slots and peel and stick it on. Put your adjuster back together and it will stay put for a long time and you will have extra product left over if you ever have to do it over.
I never saw a sandpaper used but 100 Straight and others supply a sticky backed rubber to install between plates at the time of installation. The product is available at Tru-Value and Ace hardware stores.
I've had a Jones adjuster on my mx8 for years and it doesn't have the stick on rubber tread material........but it has enough screws to tighten it down that it doesn't slip. I bought a 100 straight adjuster with only one lock down screw and after getting it installed (without putting the rubber tread material on) it wouldn't stay put.
As my Jones adjuster didn't have the material I didn't initially know what that material that came with the adjuster was for (could have had some installation instructions for a simple Swede like me). But once I put the stick on tread material on the recoil pad side of the adjuster, it locked down tighter than old Jim W holds onto his two sided coin!!!!
On the buttplates I manufactured and installed, I used 3M Anti Skid material. No one I know of uses sandpaper - it kinda looks like it but it is actually adhesive backed AntiSkid material. I liked the 3M product because the strong adhesive didn't allow the material to slip. I liked the black sandpaper-looking material better than the grayish material that was a bit more rubbery. Stays put better.