If it was me, I would just leave the towel on top, maybe set the lid of top of that if you have to keep dogs, cats, kids or whatever out of it. Makes it lots easier to stir it and you want to stir the skeeter pee a couple times a day or so. When the specific gravity gets down to 1.010 or so, then it is time to snap on the lid and put an airlock in. Good luck with it, Arne.