The Cover Creator is has the option to create audiobook covers. You'll choose Audio in the Size.
There are two square template choices 2400x2400 pixels and 3000x3000 pixels.
Here's a blog post with an in-depth how-to look at creating audiobook covers in the Cover Creator: 7 Audio Book Cover Designs and Tips to Create Them.
And here is a video showing options to promote your Audio Book. Watch Now