I had a slightly similar issue. I wanted concrete fence posts but all my ground had been dug out and would be back filled ( underground house ).
I was worried the 'earth' wouldn't be solid enough. So I shuttered the length I needed and plenty of depth. I then placed chucks of PIR insulation where each post would go ( with some allowance for play! ). From memory I wedged / screwed them in situ to the osb / scaff board shuttering.
Then got a lorry load of concrete pumped in along the shuttered trench.
Not sure if that's the best way to do it!. Once concrete cured fixing the posts was easy ( more work trying to get the PIR out ). Concrete posts have been up 4 years now - not one has moved. Hope my explanation is clear.