Your cooler needs to breathe, not stay sealed

Mold in your cooler is almost always a moisture problem, so the fix is to stop storing it sealed shut.

After every trip, wash it out with warm soapy water or a vinegar solution, then dry it completely — including the drain. Most people miss the rubber gasket and the seams. Then prop the lid open a few inches so air can move. In a damp garage, that airflow is what saves you. If you can, set it on a shelf or something so air gets underneath too.

If mold has already taken hold, clean with a dilute bleach solution (about a cup per gallon of water) and let it air out until the bleach smell is gone. Then get into the habit of storing it dry and open.

You can throw in a desiccant pack or a small bucket of silica if you want extra insurance, but it won’t fix a closed, damp cooler.

Future trips will smell a lot better.