I keep all yoyo related stuff in one case. This is a Tool Shop case I got at a hardware store. This style case is sold under different brand names depending on which store you shop at.
The dark foam came in the case. It has slits in the foam in 1/2" squares making it easy to pull out the foam you don’t want to create whatever size holes you need. I added the white foam to the lid. Along with the yoyos I have some Altoids tins that I keep random parts/junk in. The plastic container in lower right is my main parts case with spare parts and tools. There’s enough space between the bottom foam and lid foam to throw all my extra string in there.