The development of germanic warfare was well on it's way during the time when the francisca was introduced. We are talking about ages of shield making and developing successful Frankish warfare, like the development of their heavy cavalry later. Don't you think those guys would walk around with something substantial? Lamination construction is a technique that is light and strong, bow makers used it too, I believe. Those round shields weren't that heavy, 3 to 4 kg in general, the frankish ones were even lighter as they were smaller than the average ones. An axe hanging in such shield would be noticed.AFAIK There are no intact Frankish shields and there are no texts that detail their method of construction. How did this "documentary" determine their weight?