I just got these along with my microgreen seed order and thought I would show them in case anyone else is interested. These are stainless steel spring things for wide mouth jars, they hold down the stuff you are fermenting below the brine surface. They can be used with most fermenter lids as long as it is not a silicone/soft type. They looked like a good idea to me.
Link: https://www.trueleafmarket.com/products/stainless-steel-fermenting
Link: https://www.trueleafmarket.com/products/stainless-steel-fermenting