I have used tin foil on mine especially when I want to keep it partially vented because I can just fold up a corner.

Try using a dish or a cereal bowl.

A heat safe plate or the lid from a sauce pan will both work if they fit.

Foil will be fine. Don’t wrap it round too tight, make sure there’s somewhere the steam can escape.

I’ll suggest a big plate. It should fit inside the rim though so you don’t get dripping over the side, and of course not so small it falls in. I did that for a while in college when I busted the glass lid.

Aluminum foil

a plate, a pan, a cutting board, whatever is flat and big enough
also, *aluminium