Using a designer's name or brand name as a substitute for the product itself

Example: A character owns a pair of Sophia Loren sunglasses. Before going out for the afternoon, "She drew on her Sophia Loren’s, flipped her long mane back, and tossed him a cheeky grin."

If I'm not mistaken, this is an example of a "proprietary eponym". Is the apostrophe correct here, or should it be "Sophia Lorens"—i.e. since the thing referred to is a plurale tantum, should the proprietary eponym also take the form of a plurale tantum?

What if it were a Sophia Loren hat? I doubt that anybody would be inclined to say, "She put on her Sophia Loren's," or "She put on her Sophia Lorens." Despite the question of punctuation, it's likely they would say "She put on her Sophia Loren"—or, perhaps less awkwardly, "She put on her Sophia Loren hat." And, in response to "Hey what kind of hat is that?" it would likely be more common to hear, "Why, it's a Sophia Loren!"

So, I don't think that if the singular form is punctuated without an apostrophe, that the plural form would be.

Still, syntactically, it's a pair of sunglasses, and whether it's a single object that's being referred to or not, I think that rules of pluralization apply.

Normally, this would be a matter of style, and it would follow the rules of whatever style guide you use.

If using Sophia Loren as a noun, The Chicago Manual of Style would say that the plural should be Sophia Lorens.

However, in this case, there may be something else that determines how it should be handled.

From Mignon Fogarty's article "How to Make Product Names Plural":

Product names are usually trademarked, and companies don’t like you to use trademarked words generically. Making names plural counts as using them generically. . . . In other words, Apple has always said it wants you to say things like “I have two iPad Pro devices.”

Also per that article, however, "we all know that’s not realistic in casual writing and speech."

But if you don't have a style guide that you follow, and there is no guidance on the trademark of the item name—or even of the use of the company name in general—it's really up to you how you choose to pluralize it. Of if you simply rephrase the sentence to avoid pluralization altogether.