You didn't say what kind, like vertical (squared) or horizontal (rounded) channels or ribs. But, what works for all types is Aluminum C-channel
You'd just screw or rivet it to the bottom edge with one end tight to the edge and the other end about 1/8-inch off or away from the edge to create pitch and let the water drain out and not collect.
A tiny gutter. I don't know why they haven't ever come with them, for what I've seen, but it works perfect and doesn't look out of place. I think it makes them look more finished.