This was originally made to hold a Ball Marker used in a Golf Outing, but can be used to hold and disc measuring 28.6mm in diameter. 

The holder and connecting disc are designed for these magnets:https://www.amazon.com/gp/product/B0BWD84WQ9/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

And the D-ring is meant to be installed at a pause at layer 27 out of 48 which is about 56% completion, that D-ring can be purchased here:https://www.amazon.com/gp/product/B07QWJV5MZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I hope you enjoy!