Get good thick ones (subject to your taste, how much you like the look of them, etc.)
The standard rounded-edged teak (I think) windowboards, about 1" thick, are very nice, I think. You'll get them in pretty much any builders suppliers. Anything thinner will warp in direct sunlight, no matter how much sanding sealer you slap on.
(I speak from the perspective of a guy who lives in a house built by a company which must have gone to great trouble to source the thinnest, blandest, cheapest-looking windowboards this side of 1978)