Duct Tape (what else?!)
Trim the top of the bottle to that it fits your mouth. If the neck is to large to fit snugly, wrap tape around the blowgun until it fits. If the neck is too small, push it into the bore and secure with tape. (This also acts as a "safety ring" that prevents the dart from inhaled) If the neck is to small to fit the former and to large to fit the latter, get another bottle.
Try different types of mouthpieces and see which one works best.
Thanks for reading!