custom_layout.layouts.yml

custom_layout:
  label: custom_layout Two Column
  src:
    Plugin:
      Layout:
        TwoColumnLayout.php
  templates:
    layout--two-column.html.twig
  icon_map:
    - [first, second]

TwoColumnLayout.php

<?php

namespace Drupal\custom_layout\Plugin\Layout;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Layout\LayoutDefault;
use Drupal\Core\Plugin\PluginFormInterface;

/**
 * Configurable two column layout plugin class.
 *
 * @internal
 *   Plugin classes are internal.
 */
class TwoColumnLayout extends LayoutDefault implements PluginFormInterface {

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return parent::defaultConfiguration() + [
      'column_widths' => '50-50',
    ];
  }

  // Override any methods you'd like to customize here!

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $configuration = $this->getConfiguration();
    $form['column_widths'] = [
      '#type' => 'select',
      '#title' => $this->t('Column widths'),
      '#default_value' => $configuration['column_widths'],
      '#options' => [
        '25-75' => $this->t('25%/75 foooo%'),
        '50-50' => $this->t('50%/50 barrr%'),
        '75-25' => $this->t('75%/25%'),
      ],
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    // Any additional form validation that is required.
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['column_widths'] = $form_state->getValue('column_widths');
  }

}