My 2-Yr old boy is day toilet-trained.
For outdoors, I wipe the seat clean with toilet paper then let him sit on it, his legs open wide, while holding him.
Usually just for a few seconds, then it's done.
For poo, it happened once, just had to hold him a little longer.
You may also wipe the seat with a disinfecting wet wipe, or source around for toilet seat disinfectant spray, or disposable toilet seat liners.
For road trips and travels, we use a portable foldable seat cover, that can double-up as potty as well.
It's Potette Plus, available at Mothercare, to use with its disposable potty plastic bags.
Once, my boy had to poo in middle of long road trip, and it came in very useful.