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There’s one that’s designed to fit on Frontier/Spirit personal items, called Take Off Luggage.
The wheels are removable which turns it into a personal item size.
If you don’t need to squeeze it into a budget airline sizer I assume you can keep the wheels on and it would likely still fit under the seat.
Haven’t tried it so no guarantee on quality!
takeoffluggage.com
Removable wheel luggage online – Take OFF Luggage
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