I just scrounged a big round slab of a tree from a firewood seller. It is still green and about 32" across and 8" thick. My question is, how can I make it last a long time out in the weather all the time. I am using it as a knife and axe throwing target. I don't really want to spend a lot of money putting expensive varnish or wood preservatives on it! Any cheap recipes out there? Thanks, Triumphman.