When you do a timber hitch, how many wraps do you normally make? If you are used to only doing 2-3 wraps, try wrapping the excess around/through the loop until you have reached around to where the knot started (probably around 6-8 wraps), and bring the end back through the starting loop.
So, what you will end up with will be a much thicker loop, since it has been wrapped around with the extra, but you will also have a knot that tightens against your nocks all the way around the tip, instead of just on one side.
Think of it as adding enough wraps so that when the right side tries to pull out, the left side holds it in place.
So, using this one as an example, you'd leave more to work with, and make another 3 wraps or so, and run the end back through that beginning loop.
Now, I've never tried this on such a heavy weight bow, but I make all my bowyer's knots this way, and have never had a problem, up to about 80#. All of this is just a way to increase the friction holding the knot together- you basically double the friction when you carry the knot around the full loop.
Mike