Can I fix a cooler that isn't sealing properly without buying a new one?

Yes, you can probably fix it. Start by cleaning the rubber gasket and rim with warm soapy water to remove debris. Then check the hinges and tighten screws or add a washer if the lid is misaligned. If still loose, heat the gasket with a hairdryer, close the lid, and let it cool. As a last resort, buy a replacement gasket online—it’s a cheap, easy fix.