Hi Marita.
There are a lot of "convertible" furniture styles such as ottomans which make into beds, or book shelving which makes into Murphy beds, and such as that.

I'm assuming the 18 inch dimension is the HEIGHT of your a/c? If that is so, then you might consider getting....or building....a 25 inch square wheeled platform to set this a/c upon. Then you could put a "cap" on it that is padded with some cotton batting or foam sheets, then covered to make it into an ottoman. I think it might make a nice coffee table all by itself, even when the a/c is moved into the window or otherwise in use.

The nice thing about putting it on wheels is it is very movable. But to make it steady and stay where you want it, use locking casters. Look for some which can take the weight of the unit and anyone who might choose to sit on the ottoman. I think you might make this cover out of 3/4 inch plywood so it will be very sturdy. And when the a/c is in use, you might store some of your winter clothing or bedding in the empty ottoman?

I would not want to be struggling with that heavy item down stairs. I lost control of a heavy low chest going down some carpeted stairs, and it skiied its way into a door at the bottom, busting up the door. No one was injured, but we were lucky.

It is a window a/c? One house we rented had a refrigerated window unit that just slid back a little into it's stand/holder so we could close the window. It wasn't as easy as "just slid" but all the adjustments could be done from inside the house. Could you install something to hold it outside the window so you don't have to bring it in at all?