Summary

Simple variable inductor.
Main core is 1 inch diameter is a hollow tube in order to allow magnetic core or another inductor to slide in.
Need
2 x 4mm diameter aluminum rod.
3 x M4 threaded rod.
6 x M4 nut.

You will need a small piece of metal that can act as a "spring loaded whipper" (easy to find it in any electrical power strip)