Thank you for the detailed photo. Vr ball is beautiful. It was a great idea to make it wooden. I like wood, it makes things much more pleasant than plastic.
I think Vr ball can also be compared to a boundary microphone. The boundary microphone is on a hard, reflective surface, while the Jecklin disc is made of an absorption material. Boundary microphone uses the effect that an acoustic boundary layer is formed on a solid surface. And in the acoustic boundary layer, the direct sound is amplified by 6db and diffuse sound is amplified by 3db. The geometric dimension of the surface determines which frequencies will be amplified. Vr ball has a hard surface. Theoretically, a boundary layer should be formed there. E.g. if the Vr ball has a diameter of 17 cm, then the boundary layer could influence frequencies around 4000 Hz and higher. Did you notice this effect on the omni microphones? Omnis are right on the surface, there it should be the most visible.
I started thinking more about boundary layer in a connection with the dummy head, that it should also be somehow reflected there. Unfortunately I did not find detailed information about the boundary layer. I don't know if the gain in the boundary layer depends on the angle of sound incidence. I also don't know if the surface shape (e.g. sphere, plate) has an influence. These are theoretical questions, but for boundary microphones and maybe for dummy heads it may be good to know it.