A mere 3 and half years later, I have actually started trimming my rear panel to fit! I think I am going to end up with some pushing and pulling to do - but hopefully I can even up the spacing enough that any curves or flaring towards the back of the car is not very noticeable.
Part of the problem is that I am fitting the Equiniox rear panel around what is effectively 'book spec' metalwork for the rear of the car (including the rear rollbar stays as drawn in the book). It is close to fitting, even with the curved top tubes and the fact that the Equinox panel wraps over like an inverted 'U' section, but inevitably is making the whole job more challenging.
I am probably hoping to achieve panel gaps/alignment that is better than I will realistically achieve. However - this is the joy of making ones own car!