I used some screws with an agressive thread. As long as you don't overtighten the screws, the infill will be fine. The strength of the connection will come from the screws gripping into the outside layers.
Those slots in the bottom of the grip are there to attach a base plate tot give some extra support to the hand. It didn't work welk with the stock and felt weird. I left the slots in the model, since I might come up with some other kind of attachment in the future.