In your original picture, the grip is one of mine (sold through RTG) and the shortened surplus HG is done like one of mine (shortened at the front) but the wood is then smoothed down to meet the cap, which normally I don't do as it makes finish matching more difficult. Stock is most likely surplus with one of the now unavailable G3 back plates from RTG. I say "most likely" because the sling plate is kind of a pain to let into a piece of wood - it can be done but I've rarely seen it on custom work.
If you can do the metal work, making one of those back plates from a G3 or CETME one shouldn't be too difficult, then it needs finishing with duracoat or a similar product.
I like the reddit guy's work, seen it before now and then. I would have some concerns about HG durability from the way he does the ends, the Germans used metal caps for a reason. Honestly though, at that price it might be worth trying out and seeing if the HG lasts. Not everyone shares my views on durability, and more robust solution adds time and money, it's just the way of things. If you like the HG and think the risk/price ratio is reasonable, then I would say try it out.