Damp paper towel.

This is a good question, already covered in the other comments.

I will add here that exploding food has been much less common since I figured out how to choose a reduced power level on my microwave. (But I still cover the food anyway.)

I've always just used a bigger plate/bowl to cover things. They're much easier to clean than any plastic lid.

In the microwave, I always use the lid of a bigger glass bowl on top of my large, wide, glass, measuring-pitcher. It doesn't fit perfectly, but I turn the lid upside down so it doesn't slide when I carry it.

Pyrex/borosilicate dishes work pretty well here. Both Pyrex and IKEA do little single serving casserole dishes with a lid that work fantastic. Been using them for decades now

Alternatively you could upend a plate on-top of your bowl