Just to supplement…
If you want a very highbrow, poetic term, you can borrow the Greek aegis, aegidis, f. See, for example, Ovid's Remedia Amoris 343-6, where he recommends distracting attention from any unflattering physical features with jewelry:
Auferimur cultu; gemmis auroque teguntur
Omnia; pars minima est ipsa puella sui.
Saepe ubi sit, quod ames, inter tam multa requiras;
Decipit hac oculos aegide dives Amor.
We men are won over by splendor: jewels and gold can hide everything else. A girl is really the least part of herself. Often you might ask, what is there really to love, underneath all that? But with this shield of wealth, Cupid can trick the eyes.
In the comments, Tom Cotton also recommends tūtāmen, tūtāminis, n. This is a nice general-purpose word for defense or protection: anything that keeps you safe against attack. So I'd say it's exactly the opposite of aegis, a simple, straightforward term instead of a lofty metaphor. L&S's example for it comes from Aeneid 5.258-62, describing a prize given at the funeral games:
at qui deinde locum tenuit virtute secundum,
levibus huic hamis consertam auroque trilicem
loricam, quam Demoleo detraxerat ipse
victor apud rapidum Simoenta sub Ilio alto,
donat habere, viro decus et tutamen in armis.
And as for the man who came in second place, Aeneas gave him a shirt of chainmail, bound together with polished rings and triple-reinforced with pure gold, that he [Aeneas] himself had ripped off of Demoleus after defeating him under the high walls of Troy, on the banks of the river Simois—it was both a mark of glory, and a useful defense against weapons.
Finally, tegāmen, tegāminis, n is another word for "protection", from the same root as English protect. But while the other words refer to armor, this one refers to a cloak, and can mean "concealment" as well. Metaphorically, Cicero uses it to mean a solid defense against accusations and implications. It's less common than the other two, but would be just as easily understood by a classical Roman.