Question: Aren't those American vintage heads slightly OVER-sized? I know that many of the older wrapped shells were really tight fits as far as getting a drumhead to seat well and so they started making a line that would fit better on the older wrapped shells. However, a duco finish isn't as thick as a wrapped finish, so you might check to see if that makes any difference.
But, I'm surprised Rob Cook wasn't aware of contoured bottom flesh hoops on old snare-side heads. I mean, mine was a Leedy drum and Leedy must have made ALL their old calfskin snare heads this way, I would think -couldn't have been that uncommon...but I didn't ever notice it before, either. But I'm certain now that the single-flange rim that you are having problems with would be alleviated in the most original way, by using one of those old-style flesh hoops...but if Rob is unaware of them, then maybe they are impossible to get.
Your solution sounds like it works, but just in case you ever need/want to restore it to "pristine" original condition, then the contoured wood flesh hoop is the only legit solution. If you really want to get a sense of what that drum might have sounded like when it left the factory, you have to use real calfskins and nothing else. They are completely different sounding/feeling than any synthetic head. Of course they are no way "practical" by todays working drummer's standards, but they do have a thing. You would have to have Rob contour the flesh hoop though, because you couldn't file it down after the skin is stretched or you will just tear through it. Modern heads have the plastic tucked into the flesh hoop.