Waxed linen cover
[SL-WLC]

Like the product description says; it's not a water tight seal, but the waxed linen covers conform to the mouth of jars and cups like a glove and I've tied three of them as permanent lids for my drinking horns and mugs for keeping debris out while at outdoor recreations.

It has a variety of uses, as I have also cut the large waxed linen cover into strips and used as a grip wrap on a few wooden cooking utensils.