I used 4x6 PT lumber, 12' long laying flat and covered with the 2x6 plastic skins avail. at eastern marine. Usually they are mounted with little L pieces bolted to a sliding bracket. This allows the bunk to pivot or adjust to get the best angle for baring on your boats bottom. Better not to use lumber on ends (too tall) as there are several advantages to having the boat ride lower on the trailer. If your trailer is aluminum, make sure the PT wood does not touch the aluminum as the chemicals may react with it. I don't see any reaction with the galvanized supports. Pick the best Knot free lumber you can dig out of an HD pallet.
http://shop.easternmarine.com/index....categoryID=223
Cypress lumber is a very good enduring choice for boat bunks if you can find it.
Chris