As the title says, I found a perfect holder:
A Playstation carrier. Made to hold the original Playstation system that we managed to break.
It has a lid with a net.
It has three sections that can hold a lot of string, some acessories and a lot of yoyos.
Or you can put all your accesories and string in the net.
And remove the separator to get room for even more yoyos
I find this to be as close to perfect as possible. Since it is made to hold,carry and protect a Playstation, it already has some foam along the edges. And that removable separator makes it very easy to customize. It has a strap as well, so it’s easy to carry. I am planning to find some foam to stuff in it for the future, but with my amount of money and yoyos, I don’t think it’s necessary yet.