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squareplates posted on r/gowildfrontier71w
I use this: Large Travel Backpack. My girlfriend and I both have one since the pass first launched. They're durable, as good as when we got them. 12.99" x 18.5" x 7.87" technically, it may be 0.5" too long but that may include the handle. I've dipped it dozens of times in the bag checker and never had a problem. Half the time they don't even make us dip it. It's the perfect personal item sized, and it fits completely under the seat. I see a lot of them around.