I want to be able to string some temporary electric fence tape across a
stable yard which is too "solid" to take spike-type fence posts. I have
rigid fixing points at each end, but want some intermediate posts.

I have some posts which have lost their "spike" and plenty of 20 litre
containters originally used for industrial chemicals.

My "cunning plan" is to cast a thin grout in the bottom third or so of
some containers, holding in place posts inserted through the filling hole.

The grout doesn't need to have much strength, I was thinking about 1:1
OPC and soft sand, but how much water can I add and still have it "set"?
It needs to be reasonably easy to "pour" through the 50 mm filler. I
suppose I could just use wet sand.