I am glad to see the large Pet go can be used on airplanes. When I got it I was concerned it was too big.
Question when you are flying how long can the cats go without using the litter box and how do you deal with a layover situation, I am trying to imagine portable kitty litter??? How about when you get to your hotel, have you somehow packed litter and a box? this is so interesting especially as I would like to take George with me when I need to travel at least when he is young.
When I traveled (once) with my kittens who were 4 and 6 months old, we had a layover of 3 hours. I had packed one of those large rubber baking pans in the bottom of the carrier with the lightest litter I could find (my cats will use anything) and enclosed the litter in a large ziplock bag. I also had a clean-up kit with wipes, latex gloves, sanitizer, small ziploc bags for poop, and a small spray bottle of deodorizer. I also had a dog water bottle that opens so they can drink, though neither did, and a tiny amount of dry food.. Neither kitten used the litter box during the layovers. I had a larger plastic litter box packed with more litter in it inside my suitcase for the hotel, plus a scooper, and more cleanup supplies. My kittens did not use the litter box until we were settled in the hotel. I did let them use one in my car after the 2 hour trip to the airport when I arrived, but I'd say they held it for about 6 hours, and I had fed them lightly earlier in the morning before we left.
Before you travel, make sure the carrier meets the airline's size guidelines; all seem to be different. Mine was 1" too tall according to Air Tran's specs (and my carrier can zip down on the sides making it a bit smaller) but not one person bothered checking the size of it....they had never seen Savannahs, so they were quite a hit....and diversion

Both my kittens wore secure, snug fitting jacket harnesses with a leash and tag with my name and phone number on it. When you go through security, you have to take your cat out of the carrier (that goes thru on the scanner belt with the rest of your carry on things) and walk through the people scanner with them. I was concerned the metal tags and rings on the harness would set off the alarm, so security was willing to have me take one off, send it through, and as it did not set off the alarm, I carried both kittens through with their harnesses on without incident.
On the plane, both my kittens fell asleep right away (like me lol), and you do have to put the carrier under the seat in front of you, and not remove them during the flight. I unzipped the carrier to the largest size, and there was plenty of room both under the seat and in the carrier for them, and I checked them occasionally. They never made a sound, both during the flight and the layovers, but they had a few months of practice beforehand, playing and riding in the carrier (with harnesses on) with it zipped closed around the house and outside, so were used to it. They were also both used to going out in public...the pet store, meeting strange people, dogs, etc.