I don't play any string instruments, I did this pick for a friend who plays the bass, I guess this kind of picks should be used with instruments that require high pressure on the cords in order to play, I can't give additional information about the ideal pick for each person, I just can say it's easier to work with a 3mm aluminum pick and it's easier round shape with it.
You will need:
-Aluminum sheet (3mm works best).
-Sand paper (150->200->300 or more)
Engraving part (optional):
-Paint (I chose black)
P.D: Sorry for the mess.
When finished draw a cross like the one in the picture, then draw a circle, that will give you a basic idea of where do you have to remove material.
P.D: New files can be more difficult to use than old files, I recommend using an old file for this job because it won't get stuck as often as a new one, if you're using a dremel a belt sander or similar then good for you, but take care and check the process periodically because you'll possibly remove too many material and make the pick look uneven.