Amidon Associates Inc. makes a series of slug-tuned inductor forms that can be used to make any-value coil that you are likely to need. Figure 4-2A shows an Amidon form, and Fig. 4-2B shows an exploded view. 74 Winding your own coils 4-1 Variable inductor on open coil form. 4-2 Shielded slug-tuned variable inductor.