Personally I don't like the "wood block" way of doing this. It's too easy for it to slip and the tensioner then pops out of it's bore. The people who claim they will 'fall out anyway' are trying to jam the chain up high between the upper rails with this wood block way of doing it.
All you need to something that will slide down snugly between the chain all the way down to the crank sprocket. The key it two get something down at the bottom of the rail as that's where the tensioner is. You want something between the chains down where the tensioner piston is. Look at a picture of the chain layout and it will become obvious what you need to acomplish. I use a long screwdriver with a square handle end slide down between the chains to keep the tensioner from popping out of it's bore.