The cheapest way would probably be to attach the bomb to the underbelly, with screws or even glue and duct tape if it's going for a suicide mission or with a release mechanism controlled by a servo if it's supposed to return. It's pretty easy, we did it with a friend with one of those bomb-shaped toys that whistle when you throw it in high school. We attached it to an RC airplane and it worked pretty well. There probably are off-the-shelf solutions too nowadays, something like this.