Maybe someone who is not intimidated by accusations of cultural appropriation could build a costume along these lines.
In practical matters, I find the most benign way of finding some way to replicate the relics of a marginalised people is to ask them directly if it is permissible.
Peoples are not uniform in their attitudes towards their own cultures, after all. Some are highly protective, others unbothered by cosplayers.
Since there is no one uniform rule for when appropriation is appropriate, why not ask the ones most affected by it?
I note the Nias people are still in existence, and have a tourist bureau and museums.
They can probably be contacted and asked if cosplay is an acceptable use of their history.