I realized there are not many resources to find a boilerplate custom entity. Here I share a code snippet containing one. In the example below, the entity is called "example" and added to custom/modules/example/src/Entity/Example.php Make sure you rename!

<?php
/**
 * @file
 * Contains \Drupal\example\Entity\Example.
 */

namespace Drupal\example\Entity;

use Drupal\Core\Entity\EditorialContentEntityBase;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Entity\ContentEntityInterface;
use Drupal\Core\Entity\EntityStorageInterface;

/**
 * Defines the example entity.
 *
 * @ingroup example
 *
 * @ContentEntityType(
 *   id = "example",
 *   label = @Translation("Example"),
 *   base_table = "example",
 *   data_table = "example_field_data",
 *   revision_table = "example_revision",
 *   revision_data_table = "example_field_revision",
 *   entity_keys = {
 *     "id" = "id",
 *     "uuid" = "uuid",
 *     "label" = "title",
 *     "revision" = "vid",
 *     "status" = "status",
 *     "published" = "status",
 *     "uid" = "uid",
 *     "owner" = "uid",
 *   },
 *   revision_metadata_keys = {
 *     "revision_user" = "revision_uid",
 *     "revision_created" = "revision_timestamp",
 *     "revision_log_message" = "revision_log"
 *   },
 * )
 */

class Example extends EditorialContentEntityBase {

  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
    $fields = parent::baseFieldDefinitions($entity_type); // provides id and uuid fields

    $fields['user_id'] = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('User'))
      ->setDescription(t('The user that created the example.'))
      ->setSetting('target_type', 'user')
      ->setSetting('handler', 'default')
      ->setDisplayOptions('view', [
        'label' => 'hidden',
        'type' => 'author',
        'weight' => 0,
      ])
      ->setDisplayOptions('form', [
        'type' => 'entity_reference_autocomplete',
        'weight' => 5,
        'settings' => [
          'match_operator' => 'CONTAINS',
          'size' => '60',
          'autocomplete_type' => 'tags',
          'placeholder' => '',
        ],
      ])
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE);

    $fields['title'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Title'))
      ->setDescription(t('The title of the example'))
      ->setSettings([
        'max_length' => 150,
        'text_processing' => 0,
      ])
      ->setDefaultValue('')
      ->setDisplayOptions('view', [
        'label' => 'above',
        'type' => 'string',
        'weight' => -4,
      ])
      ->setDisplayOptions('form', [
        'type' => 'string_textfield',
        'weight' => -4,
      ])
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE);

    $fields['status'] = BaseFieldDefinition::create('boolean')
      ->setLabel(t('Publishing status'))
      ->setDescription(t('A boolean indicating whether the Example entity is published.'))
      ->setDefaultValue(TRUE);

    $fields['created'] = BaseFieldDefinition::create('created')
      ->setLabel(t('Created'))
      ->setDescription(t('The time that the entity was created.'));

    $fields['changed'] = BaseFieldDefinition::create('changed')
      ->setLabel(t('Changed'))
      ->setDescription(t('The time that the entity was last edited.'));

    return $fields;
  }
}