From wisebrother.com:
"A sneaky travel hack is making the rounds on social, and it's especially helpful if you're an over-packer . . .
You can effectively bring an extra carry-on bag with you, and not be charged for it . . . if you just pack a bunch of clothes in a PILLOWCASE. Some woman recently posted about it on TikTok and says it always works.
She uses a pillowcase with a zipper, so nothing can fall out. It's also got a pattern, so it's harder to see through. And bonus: It still works as a pillow.
There's a chance the airline will count it as a "personal item," and charge you if you've got another one. But one person asked her if it really works, and she said it's worked seven times in a row."