How can I transmogrify a fixed monitor post in to a variable height monitor post?
This is a bit out there and seemingly off track, but please bear with me.

We have a premium shower system which includes a shower head holder on a vertical bar:
  Except our showerhead is much more substantial than in this photo and exerts fair bit of torque on the slider assembly.  So much so that we barely have to tighten the slider.

As a result, we tend to leave it fairly loose so we can adjust it with one hand and only a little force by providing relief of the showerhead torque.  It then slides easily up and down.

Perhaps that adding a single ring to glide closely around your post could use the same principle.  It would be about 10.2 cm interior diameter, probably 5 mm thickness, and about 2 cm high.  Weld to one side of the ring a vertical bracket to attach bolts to fasten the monitor arm.

I don't expect any artistry awards, but maybe this is clearer?