Haha, yeah if there's one thing you can be certain of, it's that fans will call you out if you miss something.
The helmets are described as skullcaps (literally, I guess), so I imagine they'd contour pretty closely. Remember that the design mandate for most anything related to nature on Roshar is going to be arthropods, crustaceans and coral. If you take that into account as you go, it's easier to stay on track. But the best thing to do is make note of descriptors in the text (much easier in a searchable e-book) and draw directly from that. I think the best place to look is during the chasm scavenging scenes with Bridge Four... there's a good description of their armor and how it grows out when Kaladin tries to strip one the first time. There's descriptors of their clothing and weapons as well.
It's also worth remembering that while the parshendi look primitive, their weapons are very finely made... I wouldn't necessarily make them look too rough. They carry short bows rather than long bows, and I think a smaller hand-axe might be more suitable than a long-hafted battle-axe. Maybe look into Mongolian weapon design and extrapolate from there.