This is an overall "good" battery box that I decided to get for my GSG 522 (mp5 variant).
First things first: this thing is HUGE. DO NOT get this expecting to be able to use that rail you put it on. Also as any battery box does, it'll make your fun fairly front heavy so I recommend adding a foregrip if you decide to get it. For $13.00 you get what you pay for. It's made of all plastic and it has plastic (not glass) lense covers.
The rail mount is really fidgety, took myself about 15 minutes to finally get on. However it is actually quite stable. Just make sure not to overtighten as it'll probably strip.
The day I received it, one of the fake dial covers fell off (not a big deal with some crazy glue) and that plastic lense was shot out in one shot by a BB.
This thing DOES NOT fit a large cell battery type. Despite it's size, they will not fit. So far any type of battery will fit except stick obviously. however it might move around a bit if you have a nunchuck or other type in there.
Overall: For 13 bucks it's fine. It's cheap but that's because it's cheap if you know what I mean.
Get it if you just need a battery box for a front wired gun. Not so that it looks cool.
Slightly strong rail mount
huge = easily fits batteries
parts might be assembled cheaply
can be broken easily
doesn't quite fit large battery types
will take over the front of your gun