Coil make transformer based on customized design.
The transformer is manufactured on a sendast core, an analog of KoolMu from Magnetics 77380, with the following characteristics:
Relative magnetic permeability: 125
Outer diameter after coating: ≈18mm,
Inner diameter after coating: ≈ 9mm,
Height after coating: ≈7mm

The first winding is performed by a winding wire with a copper diameter (excluding the insulation thickness) of 0.28 mm in one layer, turn to turn, along the entire length of the core. The resulting number of turns is about 70.
The second winding is performed by a wire with an external insulation diameter of about 1 mm, in one layer, about 15 turns.

Good mechanical strength should be obtained between the primary and secondary windings, so that the windings do not rub against each other. 
The insulation between the primary and secondary windings is 1kV AC for 1min.