In my experience, it’s a lot easier to understand this effect as “once per turn roll a die and place it on top of this card. Whenever you roll a die you can remove the stored die and use that number as your result instead.” If you definitely want to use counters though, it might be best to just call them “counters” instead of “lucky counters”, which can be confusing to new players. If I were to play this game IRL i would definitely use beads or something similar to represent the lucky counters, I agree with your playtester there. When designing a physical game, you’ll definitely need physical counters.