Yeah people underestimate the smell of a rotting watermelon. It's pretty close to durian. Had a friend put one in their trunk and forget about it for a couple weeks, until like a week after it started to smell.
How about a 750ml bottle? Weighted with some sort of liquid, of course, so it goes all the way down. They're pretty high up too, so you'll probably have to lob it in. You'll with a handle of some sort, maybe made of fabric, so you can get a nice lob throw and land it right in the stack.
Ooh! One of my favorites is slapping bologna on the car. Leave it and it will eat through perfect circles. Polka dot compensation truck. It's gotta stay for a few days tho.