I'd machine to T shaped metal block that drops into the slot and the top of the T just bridges the slot. Drill a hole in the center for the screw. This will allow you to slide align the screw to the lug. I haven't had much luck bonding stuff to synthetic stocks. They all seem to be made of different material and getting something to stick to them isn't always easy.
How about some 2 part epoxy with a nut embedded in it?