Your injection molded stock is not going to be polyurethane based, it's most likely some polyolefine. Doesn't improve your adhesion issue so. Nothing really sticks to those, and they are much more flexible than the bondo (polyester) or epoxy resins you typically use for sandable shapes. I would insert wood blocks in the hollow spaces of the stock, and fill the rest with polyurethane construction foam. This should give you a good base to screw your cheek piece on. Once your cheek is done, you can use whatever you want to smooth it out, but don't try to get an overlap with the stock - it will pop loose in short order.
F 135 - the right choice