In 5e, there are three kinds of spellcasting focuses, that a Cleric can use. Amulet, Shield, and Reliquary.
A reliquary is a box that contains a relic (or fragment of a relic).
The relic itself need not be displayed. The box can remain shut.
By simply holding the relic, the box itself becomes the spellcasting focus.
Similarly, by holding the design, the shield itself becomes the spellcasting focus.
Welp, I'm off to get a root canal done in 30 minutes. Good times.
Precedence should be decided by specificity. The "worn clearly" rule, applying only to holy symbols, is more specific than the "free hand" rule.
actually they could not do so in 3.x eitherClerics cast spells while holding a shield and weapon. That's a tradition that goes back to the start of the game.
In this case, the ability to do somatic components while holding the shield, means it is still possible to do somatic components while holding the shield, whether the shield is necessary or not.