Never done that before myself, not having a synthetic stock yet (but I want one!). An idea maybe, but couldn't you just press in a hollow tube like in the middle of a paper towel roll, spray in the foam, holding it in place until it mostly sets. Of course too much foam would crush it, so you would have to be careful there. Seems a way to get access to the stock bolt and still deaden the sound nicely. A plastic tube would be better and if I find one someplace, I'll hang on to it for just such a project. 44 Man