If you want less flex in the mounted light, just rotate the DiNotte mount 180 degrees so that the flat part of the mount is on the left, not right. This shortens the moment arm (leverage arm) of the mounted light to about half the length, resulting in about half the flex.

If vibration of the mounted light is a problem for you or others, rotating the mount 180 degrees to the other side also changes the natural vibration frequency of the mounted light. Then the natural frequency of the mounted light may no longer match some frequency your seat post vibrates at.

Nice mount, by the way.