If the wheels need a spigot ring then they are not Ford fitment, which is in your favour for an easier solution along the lines you were thinking of.

Whatever way you try, if you had ford spec centrebores on your wheels then hubcentric spacer of less than 15mm cannot be done.

The fact you have now clarified your wheels don't means you have some excess width on your existing hub to play with.

All you need to do is sleeve with steel tube your existing hub to the same diameter as the larger than Ford spec hole in your wheels.

Simply also have the ford hub holes in the bell and existing spacers machined out to the same diameter so the sleeve can slide through them as well, they will also hold it in place

The sleeve would need to be 8mm longer than the distance from the hub back face to the existing end, in order to give you 10mm of a centralising bore, Job done.

Far cheaper than the route you have come up with via MK and no need for spigot rings either then.

All the centralising bore does is correctly line up the wheel in order for the bolts to be secured correctly, once they are torqued up its then done its job, the wheel does not rely on it for strength.