eg. no rocket nozzle... rather, a long tube is used, with only the top 1/2 or less filled with composition, and a weight-based stabilizer (eg. a long stick)
The purpose of the stick is to make the center of gravity somewhat BELOW the rocket nozzle. (eg. keeping the top end light may help) This is different from an aerodynamically-stabilized rocket, where the goal is to have the CoG in front of the center of pressure. TODO: Dad noted that if the nose-cap is blunt or has too much air resistance, then CoP would have to be calculated EVEN for the weight-stabilized version? How do you factor that in? ALSO, this notes that it's important to do the balance measurements BEFORE the fuel is added, since that's when it's the least stable.
ACTUALLY, I read elsewhere that CoG-ahead-of-CoP still applies to fire-arrows. If this is true, then adding fletching to the end of the stick would help.
Try to balance the rocket, and make sure the CoG is in the desired place.
Using a wire to guide the rocket at launch is one possibility. Another is just to use a trough.