With nice stiff sleeves, I cut the deck into manageable sizes, then push the bottom corner of one stack into the middle of another stack until they're riffled with each other.

I do that until every stack is paired with another, then cut the stacks and mash them into an unpaired stack.

Keep doing this until all the stacks have cards from every other original stack in them, then get somebody else to stack the stacks in whatever order they want.

Sometimes I also just leave the deck in multiple stacks (as you mentioned, Ark Nova's deck is huge, TM as well, and with sleeves can be impossible to stack high) and let people take the top card from either stack. Obviously if the top card of the deck can be manipulated then you need a rule for what happens in that case.