This is Chapter 8AC from my upcoming 4 hour Premiere tutorial.
I'm probably not gonna ever explain it in more detail than this. you have to have a pretty good understanding of AHK and scripting if you want to make this work, and to avoid the pitfalls (like figuring out exactly which pixel will look different when the triangle is open, closed, or missing.)

Key assignments: (You can use the normal function keys, rather than F14 to F18 like I do.)
https://github.com/TaranVH/2nd-keyboard/blob/b915ba12f1c97756d66f26184b5b7183993de086/ALL%20MULTIPLE%20KEYBOARD%20ASSIGNMENTS.ahk#L872

The actual function itself: (You don't need to keep the functions and the key assignments in separate scripts, like I do.)
https://github.com/TaranVH/2nd-keyboard/blob/b915ba12f1c97756d66f26184b5b7183993de086/All%20Premiere%20Functions.ahk#L1122