We made one of those in shop class in HS
Now...I am not condoning anyone build one of these rocket launchers....
And not saying that we ever built one and appprox 50 rockets and fired them at various things....
A D size Estes Rocket engine is the same size as a "3/4 aluminum conduit piping.
8" length of pipe.
Braise the ends shut.
drill hole in one end of pipe.
hmmmm, prolly too much info here....
anyway, the fin design was strips of tin, about 4" long and about 1/4" wide and bent near the top, they are then attached to the rocket body with plastic acting as a hinge..... The fins trailed out the back of the rocket until it was fired, then they opened up due to the propellant.
You also need a nose cone...
We built a launcher designed with a kill switch and connected to the car battery. Or we would have, if in fact, we actually built it.....