Pardon me for not answering your question, but I’m confused about how you will work out the contact area between the lock bolster and the side of the barrel. Have you already done that, or is that still in the planning stages? Have you already inlet the barrel with the full size of the original shotgun breech as is?
Even on round barrels, it was common to file a “short flat” just long enough for the bolster to mate up well. It must mate completely if it is to keep powder out of the lock mechanism.
Here are two photos of round fowler barrels. Short flats were filed on some surfaces, but not all the way around. And the one on the right doesn’t have a flat on the left side of the breech.
I supposed a fellow could also solder on some material, so as to build up material for a flat. But I have no experience with that.