This is a self-designed dice case I made in Fusion 360. The magnets are 10 x 3mm and I glue them in place with Gorilla Gel. This holds 3 full (7-die) sets with space for mini storage & small rolling area. The pencil that fits in this case is a Uni Kuru Toga Roulette https://smile.amazon.com/gp/product/B004OHNTVC/

A bic mechanical pencil will also fit if it is missing part of the clip.

The pictures are from an older version, the new version uses 10 magnets total and has posts to prevent the case from sliding apart.