Just something to consider, and as an up-front, I'm not a pepakura expert, but my knowledge of the overall appearance of the H3/Mk6 suit, the forearms have that "profile" about it- Large in the back, narrow in the front. I'm not gonna tell you to start rock climbing to get your forearms bigger, but take a look at the bigger picture and overall scale of the armor to your height. You may need to leave it at a larger scale than you'd probably feel comfortable with, requiring you to really fill that extra space in with foam or something similar to make it wearable.
It seems from your post that you have naturally skinny arms and I fear that you're gonna really distort the actual proper appearance of it.
Now, the alternative is if you're having trouble fitting your hand through the more ideal scaled version (like you mentioned), but if you feel/think that your arm/wrist will fit and it's just your hand preventing you from getting it on, then creating a removable portion along the front of the wrist can be something to try out. Thorssoli did this w/ his H3/Mk6 kits, where that top-side detail comes separate and allowed someone like myself with giant hands to get them on, then pop that detail piece right back in place.
Just something to consider, good luck