How do enable panelizer support for a custom entity?
Just create a PanelizerEntity Plugin in your module.
Ex if your entity type id (or machine name) is 'course'. Create a file PanelizerCourse.php in your_module/src/Plugin/PanelizerEntity/

This file should extends PanelizerEntityBase like that:

&lt;?php

namespace Drupal\your_module\Plugin\PanelizerEntity;

use Drupal\panelizer\Plugin\PanelizerEntityBase;

/**
 * Panelizer entity plugin for integrating with courses.
 *
 * @PanelizerEntity("course")
 */
class PanelizerCourse extends PanelizerEntityBase {

}