my kid threw her own out too... out the car window while we were headed down a highway... I just looked around at her in the back seat car seat and said.. "I hope you were done with that because I ain't goin' back to look for it.. "
That was that. No sippy cups, no mess.. Bottles only for water at night.. rest of the time, it was a regular cup with a straw in it.. or just the cup, nearly empty so when it spilled there wasn't a lot of mess.
*laughing* of course, I am also the mother that spread a very large plastic sheet under her highchair and gave her bowls of food to feed herself.. LOL.. the dog loved me... she got to eat whatever hit the floor.