Recently an excellent book on shields came onto the market. "Der mittelalterliche Reiterschild" by Jan Kohlmorgen, shows the development of the shield from 975 to 1350 in Central Europe, supported by studies of over 20 extant medieval shields. The book is in German, but I will list his synopsis here.
The book also includes detailed instruction on how to make a historical, combat ready shield. This type of shield is intended for reenactment combat, but will work just fine for SCA combat.
The main body of heater shield of the high middle ages consists, as far as the finds showed, of wooden planks that were vertically glued together. The shield was either slightly rounded or straight. The thickness of the main body variied from 7 to 15 mm, where 10 to 12 mm was the most common thickness. The glued edges might be cut in a angle to increase the glueing surface and sometimes the planks were pegged together.
Generally shields were covered in parchment up until 1350. After 1350 there was a change to, additionally at first and later exclusively, cover the shield with one or several layers of canvas. The parchment or canvas cover was always covered with a chalk layer and then heraldically painted. If the arms were to be executed in high relief (Engobage), a supporting structure of wood, nails or leather was affixed to the shield underneath the chalk layer.
Gold and silver armorial colors where done in metal leaf or simply through yellow and white paint.
According to the finds, the arragement of the strapping was not uniform until the second half of the 14th century. The shields were fitted with one or two handle straps and either one or two arm straps. If the shields had two arm straps, they sometimes were arranged in a crossed over fashion.
Sometimes, below the regular handle strap, was a secondary strap attached which served the rider as an alternative handle with which to hold the shield and the reins simultaneously. After 1350, due to the general reduction in shield size and the accompanying weight reduction, the straps were reduced to one handle strap and one arm strap.
A carrying strap, which was common up until 1300, seems to have disappeared by the end of the first quarter of the 14th century. The ends of the carrying strap were often attached flexibly with small leather bridges.
The handle straps, and in sometimes also the arm straps and the carrying strap, in general were made from several layers of leather and joined with two seams. However, they also were fashioned from simple, thick leather straps.
The number of attachment nails used for the straps variied from shield to shield from 4 to at most 10 nails. The nailends were either carefully peined over or simply bent over on the inside of the shield. Generally the strap ends were protected against tearing with small round or square metal washers. An alternative, simple method of attachment was to tie on the straps with loops of parchment.
Sometimes the shields were also fitted with a pad for arm or hand, but this was not very common.
Often the shields had a small loop near the shield axis for hanging the shield up on a wall.