The best way to keep french fries hot for the longest time is to serve the fries on one of the hot metal baking sheet that you used for cooking them in the oven. The obvious problem here is, I don't know, maybe the hot metal baking sheet straight out of the oven that could sear the hand, wrist or forearm of any person who touches it. Position a kitchen towel or hot pads under the baking sheet in a way that people can see them, a signal that it's hot, and obviously warn your guests that the tray is hot. You can also just make sure you have enough guests who will eat all the french fries while they're still hot. This is not a "grazing" board that people pick at over the course of a few hours, it's an "eat it while it's hot" french fry board.