I made a big A, I think they call it shear legs, out of 4x4's. I put a piece of 3/4" all thread about a foot from the top. You can make them as big as you need them, but mine were only about 6' tall or so. I put a comealong from a tree to the A frame at a slight angle over the log and used another comealong to lift the log up and slid the cart under it. I would try and get the weight balanced so the cart was supporting the log with the towing end a little lighter but not so much that the tail dragged.
