Until I saw your link to Porcelain Rocket, I have never seen one of those bags made commercially.
I saw one on a bike, it was made by the bike owner. His bike had a more normal shape for that area since he had a front derailleur (like vast majority of touring bikes) and therefore his bag did not go all the way down to the chainstays, it only went down to a point slightly above his front derailleur.
Due to the wide variety of frame shapes, you might have to make one or have it custom made.
Is that your bike in the photo, and if so how does it handle with that high of a front load?