L1 ( the primary inductor coil) may consist of a pancake, 5-8 turns of any magnet wire approx. 22AWG. The inner diameter of the first most inner winding should not touch the L2 (1/8 inch gap all the way around is acceptable), and there should be a small gap between the subsequent windings of the L1. A small piece of cardboard can serve as the base of the pancake; use small slivers of electrical tape to maintain separation of the windings as you progress around the doughnut, from the center, outward.
For a more permanent solution, consider wrapping the entire L1 in electrical tape, or explore the possibility of a plasti-dip type coating, or other poly coating.
The primary coil may also may also consist of a more heavily insulated wire, wrapped tightly around the L2, using only a single layer of turns.