Good to know that "the guy" in the hardware store is all knowing! It must be that red vest that gives them all that extra knowledge. Just kidding of course, lol.
If you are ok with hot water in a cooler then a small neoprene washer should work. I'm not sure that all neoprene material is considered foodsafe. Consider this from Wikpedia, "Some people are allergic to neoprene while others can get dermatitis from thiourea, a compound used to vulcanize rubber into neoprene which can be left over after the manufacturing process. Lead-containing compounds, such as litharge (lead(II) oxide), are used as compounding agents to prepare finished products made of neoprene, and these can have a toxic effect on human blood, kidneys, and reproductive systems."