One of the guys at work made a pair of pistol buckets for the Governor’s house at Jamestown a couple of years back. We made the last for them out of a broken yoke for carrying water buckets. The flaps are functional and work fine for keeping water out.That's just the form it naturally took.
In my research I did find a couple original examples of flintlock holsters but they were actually referred to as pistol buckets. Sort of a cup shaped out of leather and lined with something soft. These are what I found on line.
Jay