Have you misplaced your silicone nozzle cap? Maybe you just didn't get it in the package (like in my case)? Well, here you go! 

It's the perfect length and taper for the nozzle. The threads are modeled so it screws on and off easily!

It's made for the standard 280 mL cartridges (with M10x1 threads), but it might work for other sizes if the nozzle tip is similar.