Suggest you ask on benchrest shooters dot com or one of the target rifle sites, plenty of plastic there and you are likely to get the answer sought.
Personally, I'd think it easy to rasp, sand/grind to just below where you think you wish to end up and then build it proud with some 'Bondo' or similar auto body putty/filler and have at it. I did something very similar on a wood stock some years ago in making a pattern from it for the stocker to duplicate when it was where I wanted it. If the adjustment you wish is minor you might not even penetrate the stock's shell, but you won't know until you get there. Just some thots.
