When you don't have a lid that fits, you can use other kitchen items in a pinch.
We've all been there: Ugggh where's the lid?! Is there any piece of kitchen equipment as hard to find as the right pot lid? Even if you've found a storage solution that makes sense for your lids, it is a real pain to fumble around in a cabinet for the top to your stockpot or pan when you need to cover it. You might be lucky enough to own a universal lid and also know where it is at all times, but for those of those of us who dread the lid scramble, try using other things you already have handy.
A Frying Pan
Maybe you've done this one before? You can easily grab a frying pan of roughly proximate size and put it over the pot. It's not going to give you a particularly tight-fitting seal, but if that's not crucial to whatever dish you're working on, don't worry about it too much.
A Baking Sheet
Got a sheet pan handy? Just put it on top of the pot. It's a little bit bulkier than a regular pot lid, but it'll do the trick. It's an old test kitchen and restaurant secret for when the right-sized lid isn't available or doesn't exist.
A double sheet of foil works well as a lid when you need a closer fit than a sheet pan or a frying pan would provide. It's a little more difficult to manipulate than a lid, but it traps heat and moisture just as effectively.
Even if you already have a lid, parchment paper is useful to have around to ensure a close fit. Basically you can cut a circle of parchment paper and put it on top of the dish you're trying to keep covered—it works particularly well with sweating vegetables. You can put the same circle under a normal pot lid to minimize moisture escaping from your dish as it cooks, like during a long braise in the oven.