Set the product design area box

You can set the custom product design area on your product customizer widget. So the customer will only be able to add images or text to the specific area that has been set in the backend. 

Open the Custom Products page, edit any available custom product. 

On the custom product editor page, you will see the 'design area' button.

Click the 'Set Design Area' button, the popup will be showing up:

Click Enable Bounding Box to make you able to set the design area.