With my super small and light PixRacer, I also went crazy trying to tame the Z vibrations.
The wining combination for me was to place it on a foam pad and then ++add++ weight to it.
The stack is a foam pad stuck to the frame with double sided carpet tape. Then more carpet tape on top to a thin brass plate the size of the PixRacer, more carpet tape and then finally the PixRacer.
Basically, the PixRacer is so light that the wires going to it induced vibrations and the plate I added was needed to overcome that while dampening the frame vibrations with the foam.