Common sense dictates that the shield itself becmes the holy symbol.
No, it doesnt. My Mom does restoration and most of the work she does is religious art. The only part that she or the people she deals with consider the religiously significant portion of the objects are the characters and designs that the work depicts. The Canvases behind the work or the material making up the piece is irrelevant and is not considered part of it, and this includes statues where the holy symbol is the whole damn thing.
In other words, you are Seeing a tree and thinking its a forest.