Never believe anything is smell-proof. Every material is porous and will eventually permeate a scent cone for a K-9 to detect. The best you can do is minimize the smell.

Place your stash in a sealed plastic baggie, buried in a jar of peanut butter, in a cooler. This will tremendously slow down the rate at which the odor permeates. A jar of peanut butter contains an aluminum paper-backed safety seal - peel it off carefully and add super glue around the mouth of the jar and reapply after content has been stashed. This will make things look more inconspicuous. Also, make sure you have other cold food items in the cooler along with the peanut butter - jelly is always a good choice. Placing lunch meat in the same cooler helps confuse a K-9 team because any interest from the dog appears to be because of the meat.