No offense but that does not seem practical to me, looks like it's pulling your whole plate carrier down on one side and how would you go prone without being uncomfortable? Also, doesn't it block your arm quite a bit?
IMO mp7s are too big for holsters. If you have it on your thigh it's going to flap about when you run and if it's on your waist you will have uneven weight distribution which is a problem for most battle belts as they rely on even pressure to stay tight on your body.
Also, in airsoft the performance advantage of a GBB Mp7 over a GBB pistol (hi-capas, full auto glocks etc) is minimal to non-existent.
I get running one just because they're cool, though. And in that case I'd just throw it around my back on a one-point sling. Not great for rifles IMO but they're pretty good at keeping things tight to your body, it will flap about a bit but at least it won't pull anything down and if you're sniping you can always set it down to one side.