This simple design uses the filament box itself to accommodate the cards at an angle. It's better to make or buy something more proper, but if you're a beginner like me, it's quite handy and doesn't take long to print.
 

It uses the box fins as separators. Use scissors or a hobby knife to cut them, then insert them into the printed part and place it as you prefer.

I know it's a very small and easy design, but it may help someone :)