In a proud and solemn moment finally cut out the heat shield material to cover my fibreglass flanges, apply it and fit the wheel, AND THE BRAKE ROTOR IS TOUCHING THE HEAT SHIELD. F&%K! Obviously my sums had gone horribly wrong somewhere...
Decide this time on a more empirical approach. Make new templates from the existing flanges, then cut them off. Make a new form, spaced accurately from the installed brake rotor 6.4mm with plastic packers, plus the thickness of the 3mm ply form.