I just did mine and used red oak with a marine spar varnish made by Behr. Put five coats on looks fancy...and the wood only cost me $125...bolt kit $40, marine spar $32, repainted my bed rails though, spray paint $15. Cheap compared to a kit I think...took some time to apply and sand the boards as I put 5 coats on them. Only problem I came into was one board had a slight warp and this made it difficult placing the bed rail in, so be sure your boards are extremely straight. You must also make sure your box is square otherwise trimming boards after placing 5 coats on is not nice!! 2 People is a must when tightening bolts for the bed rails, otherwise frustration sets in very QUICCK!! And yes I redid my exhaust and painted the frame while the box was off...
