A quick guide to strapping your shield the Sir Ysambart Courtin way.
The rear of the shield consists of two parts: The shield box, and the sling.
The hand needs no extra protection. The forearm may or may not have armour. The sling accommodates either option.
The bare hand goes through the sling and grips the shield box.
The shield box is made of mild steel. It is the last remaining part of my original armour still in regular use: this box has had over twenty years of continual use. I believe there is no commercial availability for this item. You must be, or find someone, skilled in basic metalwork. The bar that the hand grips is squared tube. It is set so the back of the hand is just off the wood. No padding required - the hand never touches the shield. The box is held to the shield by three bolts.
The sling is two thirds the size of my forearm, and is sized to allow my forearm to fit in even when wearing akerton and full forearms. I used to use regular thin straps with buckles, but found these irksome to replace, and annoying to buckle. I have replaced the sling strapping only twice - the sling has served me nearly ten years. The sling is glued and stapled down, and then the staples are covered by the steel bar to ensure they cannot be removed, and cannot come loose. The top and bottom steel bars are held down by two bolts each bar.
Finding the balance takes time - I suggest only using temporary attachments until you are super certain you have the balances correct.
I hold the shield by the leading edge, and let gravity take hold. I then get a helper to draw a plumb line. The shield box goes evenly down the plumb line.