Don't use lumber... if treated, it can be deadly, if untreated will rot fast and roots won't adhere at any rate being too smooth. An ideal mount has rough bark firmly attached to the wood (lots of texture) and resistant to rot. I don't know what trees grow near you, but if you have friend or neighbor with something that looks reasonable meeting those criteria, go for it. I have used tree-fern trunk pieces (with the fibers removed or weathered away) to good effect with Catts, but don't know if those grow near you. Look at what might grow nearby.