wicker training shields

[Tarbicus]

In which case, how does it weigh twice the weight of legionary scutum, which is made of layers of wood with a rawhide covering and a bronze rim?

I'd doubt it was the same thickness as the traditional scutum, the point being to build muscles in the appropriate areas. I guess the thicker you make the wicker shield the heavier it will be. So if you intend for it to be heavier for training and muscle building then you simply make it thicker.