No idea where to start in terms of making a fixture plate for these cases...

Vice is easy as I just chop out of the soft jaws the profile of what I am holding.

I have a feeling that this cheap vice is my major problem but also finding the work edge too.
I am now working from the center of my stock/part and using the external sides and halving them to find it, but I am finding it is still off a bit. I think that is more down to the inaccuracy of my probe setup if anything.