Originally Posted by freisste
Just close it
An airtight seal is not needed for fermentation. A sealed lid is needed if you use an airlock to check fermentation or to install a blowoff tube. A loose lid is needed to keep out nasties and will not cause any problems as long as your brew doesn't overflow and push the lid off the fermenter due to not having enough headspace.
I do not use an airlock and have a snug but not airtight lid on my fermenter. It has a ton of headspace and the CO2 layer produced during fermentation keeps the beer from oxidizing.