You can set the custom product design area on your product customizer widget. So the customer will only be able to add image or text to the spesific area that has been set in the backend.
Open the Custom Products page, edit any available custom product.
In the custom product editor page, you will see 'set design area' button.
Click 'Set Design Area' button, the popup will be showing up:
Click Enable Bounding Box to make you able to set the design area.