A friend of ours uses garbage bags and water to seal his kraut. He puts one bag inside of the other. The first bag is put in the crock and pressed against the kraut and draped over the sides to seal out the air. He puts water in the second bag, ties it with a twist-tie, and places it in the crock over the first bag. He makes sure the water fills any air gaps. The weight of the water helps, too. I guess the purpose of using two bags is in case the water leaks.
My husband bought me a fermentation jar that I just love. It has a ridge around the top that the lid fits in. After you put the lid on, just fill the ridge with water to seal out the air. You can see gas bubbles escape as it ferments.
Good luck with your kraut!
I should have said groove instead of ridge. Duh.