When a square text frame won’t do, I need to create a custom text frame. I use inDesign’s pen tool to create a box that fits into the image’s empty space, then use the text tool to add text. The pen tool works like the Photoshop and Illustrator pen tools. Just click the outline of the text frame you need. You can use the selection tool to grab a node to tweak the shape. It’s a simple task, but before I googled “polygonal textbox indesign” and found a solution, I struggled with trying to select and drag out the corners of the default rectangular text boxes. It’s amazing how little I know about inDesign. It’s even more wondrous, as ignorant as I am, I still know enough to generate a PDF that can produce a Kindle e-book.
The blue line shows the outline of the text box. The outline doesn’t appear in the exported PDF.