Liquid Nails® with plastic Airsoft® BB's.
Mix in a paper cup so that the BBs are all coated and sticky but not "floating" in the liquid. There needs to be enough BBs so that they are touching each other in the mixture. Use an plastic spoon to get them in there (mask off around the end of the stock and work clean. Finish slightly proud and squash when putting the butt pad on. The Liquid Nails and BBs mixture is not heavy and does not shrink. Don't use the BBs that are coated with colored chalk, the chalk acts as a release agent and prevents the BBs from sticking together properly. Be warned, it's permanent.