I have blown eggs before, and you have to blow
hard. It usually makes my head hurt. When I was in El Paso, Latinx people made them into
cascarones by filling them with confetti and putting some paper over the hole. Those were meant to be broken, though. I'm not sure there is any way to make them stronger except coating the outside of the shell with a clear lacquer or varnish- not exactly non-toxic. If you say filled the shell with something and then the shell got tapped it would still crack and eventually peel off the whatever it was filled with, at least that was my experience when I tried.