These are the two versions of my card holder I've been using in the last months.
This thing can contain a magnetic card + paper + transparent pvc, with a maximum dimensions of 6x9x0.2 cm, which fit all the standard-dimensioned credit cards.
Finally I've choosed the version with the central hole for estetic purposes, but the later-holed version is a bit stronger.
I suggest you to print in PLA or PETG, in order to avoid printing issues due to limited thickness: reduced plate adhesion.
-- PLA- PETG
Do use brim.
If using a cartesian printer, be sure to oriantate the longer part (~6cm dimension) with the y-axis, to avoid layer misalignment (or detachment ) when the print grows, due to inertia.
If using delta, don't care, you are just doing right ;)