I'd suggest a good epoxy filler for the inletting gaps you want filled. I'm working on a pattern stock right now and will probably limit using Bondo to adjust the external contours slightly. Someone more knowledgeable might weigh in on the chemical compatibility issue, but I suspect that the epoxy would be slightly more compatible with the stock material than the Bondo. You can add powdered metal or chopped fiberglass to the epoxy for more strength (if needed). I've used AcraGel on the internal part of the pattern stock and it sets up to an almost rock-like hardness over the course of five days.
I frequently mix sawdust with one of the quality wood glues (say, Titebond II or III0 and build up outside contours with that on wooden stocks. Cheaper than epoxy for large areas. Recoil lugs, barrel & receiver bedding, pillars - only quality epoxies.
