Anything Norse religion or viking. Runes and artifacts would be great.
But if they're striving for the worship of a god rather than historical accuracy, why is it bad that they make up their own rituals? If anything, it's the making of culture, surely...
I agree that some neopagans certainly do that, but there are multiple organisations aimed specifically at not doing that. Now yes, there are admittedly relatively few sources for these religions to be able to accurately reconstruct worship, but the worship that is being made anyway is more symbolic than anything, and just because a ritual is old doesn't make it more correct if that's the way one goes about "contacting" or experiencing ones own god.
Organized neo-paganism always gets cringey for me. I don't know why, I just find that it always gets over-dramatic with other people. Probably just what I've seen though, I'm sure there are groups that don't do it so silly.