I had a similar situation with a very hard plastic buttplate.
After fitting the plate, to get the grooves, I screwed it to a scrap square piece of wood which I could then work freely on mounted in a vice. The square piece of wood was so that I could use a set square later...
I then marked out lines with a set square and a sharp needle file point. I then used alternate needle files to deepen the lines until I had the desired grooves.
I then polished the plate with fine wet and dry down to 1200 grade lubricated with a liquid polish.
perfect result!
Jonty
