[ARCHIVE ANSWER]
The oily nature of peanut butter drastically slows the permeation process, and the sheer mess of digging through the goo also makes peanut butter an excellent choice.A new jar of peanut butter contains an aluminum paper backed safety seal. Peel the
seal off carefully, add super-glue around the mouth of the jar, and reapply after content have been added to jar. This will make things look more inconspicuous.
Also, make sure you have other cold food items like bread and jelly in the cooler with the peanut butter. And maybe add some raw meat in a bag so that if the sniffer dog signals and you get searched, maybe they'd think the dog was interested in the meat.