This is a great project to get your feet wet designing and making your own 3d models for print.

Since its a functional print the first thing you need are the critical dimensions of the model. Outer diameter/size and inner diameter coin size. What are your tolerances for the lid? What are your tolerances for the coin cylinder. All design factors.

The closest thing I found in my 5 minute search was: https://www.thingiverse.com/thing:4489799

What software are you using to design the coin holder?