First, a part must be designed to join the secondary coil onto the topload. After several revisions, I came up with the following design. The piece acts as a "plug" for the coil form with a single attachment point for a nylon nut

Second, a part must be designed to form the primary coil. I needed to be able to adjust the height of the primary in the event I needed to adjust the magnetic coupling. Two parts form a "collar" around the base. Several pegs are inserted into the collar to create the coil form. The pegs can be easily reprinted with new holes to adjust the height

At this point, the project can be assembled