as was mentioned in a earlier post a tail or streamer would work . It works by increasing drag ( this method is called drag stabilization ) . you can also do the same by tying a piece of string around a foot long onto your glow stick ( for best results you should try altering the length of the string ) .
but I would also try putting a weight on the front , and a streamer on the back . the weight on the front would help to offset your lost range from the drag caused by the streamer ( string ) .
Or you could try putting a stack of washers on the front with some kind of adhesive . make they are the same size as the barrel or smaller so they fit in your barrel ( I use a 3/4 inch barrel to shoot glow stick out of my pneumatic ) .
My thoughts are subject to your own interpretation . But if you have any questions feel free to send me mail to firstname.lastname@example.org