What can be used to cover top of air conditioner?

I used a large garbage bag. It slid right over and all the way to the bottom......

You can use a tarp

If in use, do not cover it. That keeps the fan from circulating air, freely and could burn up your ac. If not in use, a tarp or sometimes you can find grill covers that will fit over the unit.

Even if not in use, i would not cover it, that will trap moisture and cause rusting / corrosion. With out covers the air will flow thru.

The covers will also provide mice and insects a sheltered place to nest.

What does the manufacture tell you to use ? They didn't ?? That should tell you something.

Google "Covers for Window Air Conditioner" and there are all types.