Accepted plural form of "Hijab"

If we look at usage, according to Ngram hijabs is the more common form.

"English has borrowed words from nearly every language with which it has come into contact, and particularly for nouns from Latin, Greek, Hebrew, and French, it has often borrowed their foreign plurals as well. But when loan words cease to seem 'foreign,' and if their frequency of use in English increases, they very often drop the foreign plural in favor of a regular English -s. Thus at any given time we can find some loan words in divided usage, with both the foreign plural (e.g., indices) and the regular English plural (e.g., indexes) in Standard use. And occasionally we’ll find a semantic distinction between the two acceptable forms, as with the awe-inspiring Hebrew cherubim and the chubby English cherubs."

(Kenneth G. Wilson, The Columbia Guide to Standard American English. Columbia University Press, 1993)

Hijabs sounds reasonable. While I don't see it common to pluralize new foreign words as in the original language, you can always circumvent the problem with hijab scarves, signifying that particular veil headscarf (حجاب), as opposed to a Russian babushka kerchief.

Of course we do use established plurals like data and alumni.