If you ignore the concavity of the shield in your picture, it's about the size and shape to be a targe, a variety of Scottish shield that was used from all the way from the 13th century, when it was derived from the Viking round shield, to the Jacobite rebellions of 1745, after which it was banned by the Brits. In terms of carrying it when not in use, you had several options. This picture shows it slung over the back, while this one shows it hooked onto the belt. Either one could be considered the "proper" way; use whichever one feels right.