You could try a few different things to add weight. Maybe some lead shot in a baggie, or even a couple tubes of BB-gun bb's. If you have a lot of spent pellets in your trap, you've got lead. Another option is some modeling clay. I would stick that in a baggie, as well, since I don't know how messy it gets, especially on a warm, sunny day with that black stock. Rolls of pennies, you name it.