You don't need to reinvent the wheel here folks, I worked in the woods and numerous saw mills for years as a kid in Oregon. We cut plenty of cedar fence boards, bolts and shakes and nobody ever "treated" them with anything because it's completely unnecessary. That's exactly why roofs, fences, and planters are made with cedar to start with, it is naturally resistant to rot and insects. Cedar chests are used to store clothes because bugs cant stand the stuff. A cedar post stuck in the raw dirt will last five tines longer than one cut from oak fir or pine. There is nothing wrong with using a cedar fence board to hold a salmon over a fire. I have seen hundreds of salmon lashed to cedar boards leaning over the fire pit at Indian Pow-Wows. The Indians didn't use any special boards either, they just buy them at the lumber yard like everyone else. 1 X 6 X 6 foot fence boards with a notched end, that point makes them easier to stick in the ground. If you want to try the Indian method of smoke cooking a salmon on a plank, there is no need to get fancy about it.