Line the bottom of the carrier with incontinent pads so that any liquid is absorbed immediately. These handy pads usually also help to contain any odor, and it doesn't hurt to have an extra one or two in your carry-on luggage so that you can change them if necessary after you claim the cats at your destination. Using duct tape, tape a small litter pan in the very back of the carrier. If you can't find a small enough litter pan, use a tin foil baking pan. Always put the cat's bed or blanket in front of the litter pan for the cat to curl up in.
Carriers come with bowls that clip to the front door.
Have two - one for water and one for food. You can put a small amount of food in one bowl, and pre-freeze some water in the other. By the time your cat wants a drink, the ice will be thawed sufficiently. Tape a ziplock bag of dry food to the top of your carrier. If the plane is stranded somewhere, staff will be sure that your cat is fed and watered.