After watching the video, and i have played both airsoft and nerf for quite some time, that must just be some cheesy toy or an un-released gun, because the barrel on that is too long for any make of airsoft grenade launcher. So it might be a modded airsoft grenade launcher but i have my doubts that they would spend 200+ on it then mess with it, but you never know.
I love this gun, its looks pretty cool, other than a few cosmetic things, to me it looks like you used hand paints. i would just say grab can of black spray paint (or a bright color for safety) and a few things are crooked but this is much more adventurous than anything i have ever done. i commend you
Im not very creative so here are my two suggestions, buy a new gun or try and manufacture an identical peice out of plastic, sorry for the vaugness ive never worked with plastic before, but i do know many of the members have, so they might have more helpful insight. good luck!