Another trick that can help is to divide the secondary in several sections, each with its own rectifier, as I have done in my original design. Corona appears at places where high AC fields are present, that is also where most of the capacitive energy is stored. So the same measures that reduce stray capacitance problems also reduce corona problems. In the single leg design of the transformer, corona will tend to appear inside the coil, between the coil former and the core, and if the winding is too high, at the place where it comes close to the other leg. In the two leg design, corona will appear only in the middle, between the two windings.
Corona can also appear between the two coils of the choke.
To fight corona, I have in both cases inserted PTFE sheet between the coils and filled the remaining space with silicone. It is important to fill all the space, since corona will appear in air pockets and eat away at the insulation. Because of the parasitics, the capacitors tend to peak charge if there isn't enough load current. Therefore, I will change my design so that for the helix supply, where the current is low and the regulation is important, I'll use a fixed duty cycle converter with a pre-regulator.