The best info I can recall on the subject was Hokwongwei’s checklist before he left Taiwan.
He was travelling with United with this restriction on the dimensions:
https://www.united.com/web/en-US/content/travel/animals/in_cabin.aspx
The maximum dimensions for hard-sided kennels are 17.5 inches long x 12 inches wide x 7.5 inches high (44 cm x 30 cm x 19 cm).
Hokwongei obviously found two of those. I’m going through the post to see if he mentioned where he got them.