Hi Buzz: What you're looking for would be a trapezoid powder flask. There are loose copies available mainly in England (and they are indeed loose); there is also an all copper version I've seen available in the US, but it too has a very "loose" historical place. Custom built correct ones are very expensive, $450 and up if a maker has the time to construct one for you. American ones differed from the English in that they were simply wood-sided (often with curly maple) as opposed to being covered in felt or leather. They also often have tell-tale scalloped sheet iron trim and a simple stoppered spout in antler or brass (as opposed to the English levered spout). The American ones look really neat; I believe there are extant examples in Plymouth and Deerfield, MA and in Jamestown, VA. As far as anyone can tell, the American powder horn form wasn't prevalent until into the 18th century.
Smollett