I used to keep 3 GATFs, 2 VATFs, sold them all at around 14" many years ago. Got a baby GATF that accept pellets so I kept it. It's 8" now and growing.
Anyway, what I used to transport all ATFs I sold are around 33"/ 24"/20" (or 3X size of the fish) black plastic container box, lid closed but not tightly, water from old tank, battery-powered O2. Also asked the buyer to ready their O2 too. All of them survived and grew in their new tanks. Hope that helps.