A few things to consider:
The main tube looks a litte bit detailless. Maybe add something to it like
GS-bands or a handgrip on top of it. Or maybe some "stock" that fits onto the shoulder plate and locks the weapon in place.
Also you maybe could add a more sqare element to the side on the front similar to the real weapon.
Also they are rather thin compared to normal
GW missle launcher bitz. Maybe use a thicker rod? Yes i know that the real stinger isn' that big but it would otherwise look underpowered compared to normal launchers. You don't need it to be as thick as the original but just a little bit thicker than yours.