I don't think anyone considers their layout a secret except the very few cases where people are selling cabs.
Most people just start with the size they want the control panel to be, then experiment with the layout to figure out what feels right to them.
They assume someone else building a cab will do the same because they most likely will not have the same cab width, viewing angle, players might be a different age (therefore size), etc.
If you search the forum for 4 player layouts, you're bound to come across discussion on not angling joysticks, Malenko's joysticks in a straight line, aircraft carrier hate, etc.