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The guy from http://www.inuvik.es did both @CarlosBI and my framebag, but he's currently doing the Tour Divide with his wife and son.
There must be someone in the UK that knows how to sew layers of Xpac and velcro! Just give the correct measures, using a cardboard, and specify that it should not be wider than the frame tube, and with that velcro in the middle to secure both bag walls. Now that I've some experience with it, I would have asked for two of those internal velcros, to have 3 separate compartments, and those inward dragging points.
Sounds like you possibly should get rid of it. Mine was very annoying when I started using and it then just became possible to pedal normal later. But I can well imagine that it could impede your pedal stroke.