I gotta go with the "intent counts" side of things here; skin/coin contact isn't enough, or they'd just have laid the coin on Harry's skin when they had him. Ditto Marcone; ditto Ivy -- the Denarians had each of them, and in all cases *could* have forced skin/coin contact. For some reason, it wouldn't have sufficed (for Denarian purposes) in these cases.
I presume, "intent" is they key. I presume that no Shadow-Denarian *can* gain a Host, if said Host is physically forced into contact with the coin, despite their intent to avoid such contact by all means possible.
Thus, a Crown Royal bag is "sufficient" for Harry -- it symbolizes his "intent" not to touch or otherwise "take up" the coin. For someone Faithful, obviously their "intent" is better shown with Blessed cloth, than a scrap of booze-bag.