What kind of wood?
Out of curiosity I did a quick internet search for Italian polenta boards and of course I came up with a lot of recipes and site describing serving polenta on a board. I only found a couple with boards for sale and they were both round and sold as a combination pizza wheel and polenta board. They just said hardwood so I would guess that it doesn't matter too much which hardwood you used however I would stay away from oak as it ususally has a wider grain than others. If it were me I'd use either beech wood or maple.