As I've been asked a few times about my signature custom font tail registration a few times, I've put together a quick PDF guide on how to do it so you can do it yourself!
For the panel.cfg file, there is more details at the official SDK page located here (font color, scaling, etc.).
Also, take care to note which vPainting number corresponds to the exterior tail registration for the aircraft! They vary from plane to plane. You will need to plug in the correct number in the panel.cfg file.
CREDIT: Crispy136 (aka BilingualHarp7 on MSFS forums) for originally devising the idea and making it work. I just took it and went with it. All credit should go to this guy (his original post: https://forums.flightsimulator.com/t/how-to-change-font-in-a-registration/325317)