BB, I make alot of hard cast leather items, ball bags, canteens, tobbaco canteens, etc, not sure if casting a powder flask would be the ting to do, no matter what you do to leather, it has a tendency to attract moisture do to the fiberous content of the material, even sealing the interior of it with beeswax will not inhibit the moisture attraction, there is a food grade latex product that can bee used to seal horns and leather that may work enough , could be interesting to see if it works well enough to keep the powder from caking, not sure of the name of this product, but I was told that it can be purchased at Michaels crafts and possibly Hobby Lobby, if you pursue this and build it, let us know the outcome and performance, Thanks, Beav