How to remove a 40mm shower head without extra tools?
If you have a thin leather belt of sufficient width to engage well the fitting, consider to wrap it around the fitting as snugly as possible. Leave sufficient material at each end of the wrap to secure the ends against each other. 

One way of looking at it is the leather will form the shape of the letter P. The vertical bar of the letter P is where to attach your existing wrench. Place the moveable jaw of the wrench in the portion that will contact the fitting when moving the wrench. As you begin to apply torque to the wrench, this jaw will compress the leather against the fitting as well as providing better leverage for releasing the fitting.

Obviously, you will want to tighten the jaws as firmly as possible on the leather strap ends. As you begin to apply force to the wrench, you may be able to further tighten the moveable jaw even a small amount, but that may also require three hands.