A decent suggestion from NickAu, only one little thing worries me, probably best not to use a new piece of wood thats just fallen from your tree in garden for example, am not sure if ANY old wood or just certain types of wood is safe to use in fish tanks or best with dead wood that has been dead a while as reckon new wood would still have sap and resins that may effect the tank etc
Once you know the wood is safe, obviously must be cleaned thoroughly.
I like the idea, be good if anyone can pipe in with an answer with some backup of research/study if possible as I would be keen to knwo this for sure.