Treated wood is what I've always used and seen used. 2x6 would do fine as long as it fits between the ribs if it's a metal boat, no problem with glass. Plastic is also available, but I believe it would be prone to sag. If you have a metal boat, you can also use plastic sliders that go on top of the bunk and your boat will slide easy beyond belief. You can not take off the bow rope when backing down the ramp or you will drop the boat on the ramp causing big damage. That said also know that loading is a breeze with no drag from the carpet. Pick your lumber to be straight and with as few knots as possible and it wont twist or warp as much as the knotty stuff will.