fireworks driven model rocket

I have made this rocket designe to be used with the propellants of a normal firework rocket. the engine should have an outside diameter of arround 21mm to fit snuggly inside the tube.
The rocket flies great, when you set the center of gravity correctly. To make it easier to you I have made a little notch, where the rocket needs to be balanced on. You can hang it there with a string while gluing wheights into the tip.