I wonder if perhaps it should be a "golden rule" when using rod type formers that whatever insulating material is involved the structure should be a tube rather than a rod. For most practical purposes tubes will be almost as rigid as rods but will be lighter in weight and by definition less dense than solid rods. Even better with "basket-weave" coils might be schemes which allowed a suitable adhesive to hold the turns in place and the former rods to be withdrawn. As a further contribution, how about the use of glass which may be obtained from specialist suppliers. It's possible that someone whose hobby is glassblowing and glass sculpting might be persuaded to produce bespoke formers also.

 

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