I have created a paint tube key. It was designed for my wife's watercolor paint tubes.

This fits a few different size paint tubes 
LG  (21ml) or (0.71oz)
Med (14ml) or (0.47oz)
SM  (5ml)  or (0.17oz)

I have created this in Fusion 360, One has an opening is 30m x 2.5mm and the other has 34mm x 2.5mm. 

It's easy to modify my model so if you need a different length send me a message and I will try and make it for you.