I just posted a question and promised myself I would answer one in return so here goes, there is a lovely Quora article wrestling with this same existential question. ;) https://www.quora.com/Assuming-McDonalds-is-spelled-that-way-how-do-you-form-the-plural-possesive-McDonaldss-Or-McDonalds
😂
That ends with:
"the best approach is to re-phrase the writing so as to avoid having to use the plural"
That's my go to for anything I don't know how to spell. Lol. Makes me seem smarter than that which I am:)