Whenever I wet form leather, I let it soak until the bubbles stop coming up (time varies), then put it in a zip lock bag over-night. Then I put the item in (usually a wooden mold of the item...in this case I'd 'donate' some loaded shells to the cause and shoot them up before they rusted from the wet leather/tanning compounds) and leave it there until the leather dries. This will take several days, but the leather will mold to size in a rock hard fashon. Your duct tape trick may work out for this process.
This process works best when constructing leather items/cases (when you can nail the "raw" leather around a mold), but still works pretty darn well on already constructed/dyed items.