Question: instead of array, couldn't we also have used the Particle System to simulate the coins?
Array modifier seems to add more objects into this model which may sometimes be unwanted. A Particle system applied to the coin seems to make a bit more sense if we don't want to unnecessarily add objects to this system. So between particles vs. array, which one should be used?
Also to fix the issue of coins falling straight through the box, add a solidify modifier to the box. The simple paper thin box is not "thick" enough to hold the coins, A solidify modifier with thickness of 0.15 was enough for me to fix the issue of coins falling through the chest.