Hi Callum,The only surviving "heavy infantry-shields" I know about, are the real large pavises, they have at the Angermuseum at Erfurt (eastern-Germany)1350 and those at the Historic Museum at Bern (CH)14.-15.th Century
They are as tall as a man and half as large (that means: ca. 1,8m x0,9m) ;O)
They (the Berner shields) are made of Pine-Boards ca. 10cm wide 2,5cm thick, glued together(probably with dowels) and thin Oak-boards ca.7cm wide and about 5mm thick glued crosswise (horizontally) on the outside of the pine-construction. The whole construction is curved from one side to the other.They are covered with a Layer of fibres in Bone-Glue, and Rawhide all arround.
They have several straps for holding and carrying them and some for fixing them on "beams" to form a kind of walls.
The Straps are made of "sämisch" leather (the stuff indians used too ;O)) fixed with huge iron nails coming through from the outside of the pavise and are hit back throuh the leather in the body of the shield.
They are painted on "gesso" in the colours of the city. (Erfurt has a Silver Wheel on red Ground. The Wheel is really made of sheet silver on the Erfurt-P.!!! )(Bern has a black Bear on a red, gold, red diagonal ground)
P.S.: excuse my poor english ;O)