Years ago, I bought some third party dual GoPro charger off amazon.
The first thing I did, was take it apart... It was very heavy and I was interested what I would find inside. To my surprise, most of the weight came from an steel piece, which was put inside the case. It looks like it was deliberatly put there to keep the charger from flying around or to give it a "proper feel". I don't know...
The charging circuit was also quite interesting. It only consists of the reference circuit to an UN8HX charger - which is a cheap 4.2V lithium charger IC with 500mA charging current, which was very easy to reverse engineer.
One funny thing though was that the two charging ports were connected in parallel to the charger. In my opinion this is not very good for LiPo batteries, especially if you try to charge to cells which have different charging levels.
At some day, the charger stopped working - I'm not sure why and I do not want to know... I bought some cheap LiPo charger circuits of amazon, in my case some boards which use a TC4056A (a 4.2V/1A lithium charger IC). After some sawing and glueing, I ended up with a much much smaller and better charger:
I also added a small spring on the side, as the charge pod size would not properly fit the batteries, hence they would fall out of the charger if used.
The charger works much faster than the original one (the original knock-off one). The biggest advantage though is the size: The old charger was quite clunky and much bigger than the GoPro itself.
(I know, the sawing is a little bit nasty and the thing does not look nice... I promise I'll 3d-print a proper case if I ever have time to do it ;))