My auld buck also recommended that we put this into the floor when we building the house. So I did, all it is is a piece of plastic piping (2''). As it is, each end is closed, but I would just have to break out the floor of the hearth and put a vent on the outside.
I don't think one would have to get a particular type or make. All it does is allow air to be drawn up into the floor.
Cast iron sounds like a better idea alright, but I doubt that my plastic pipe will combust while enclosed in concrete.
We live in a pretty windy part of the country, and my auld buck suggested it as he reckoned that depending on the prevailing wind, the fire place could smoke a lot (just like the house at home, and the AB wished he had done it).
I suppose that the logic behind this is that when the fire can't draw enough oxygen onto it while it burning, especially when there's plenty of fresh fuel, and the doors are closed (always had to have one open at home)that the smoke will sometimes come back into the room when there is a downdraft, but having the vent underneath allows the fire to draw air up through it anyway.
Can also be retro fitted I suppose, you'd just have to chase a channel out of the floor between the fireplace and the external wall, and put a pipe in. Could also put it above the floor if you had units or that between fireplace and external wall.