3 years after printing the last back scratcher, it became clear it doesn't need any removable parts.  The life of the scratching part is going to be as long as the life of the PLA. All the effort in the clamp mechanism went to waste. There were also some ideas for a novel scratcher shape.

The trick is the scratcher has to be shaped like an L. It could be offset.  The other trick is the scratcher needs to bulge in the middle.