Drupal 7, Panels 3.3+ Pane Style Plugin

accordion.inc

<?php 
/**
 * @file
 * Custom panels. Place in MYMODULE/plugins/styles folder
 */

// Plugin register
$plugin = array(
  'title' => t('Accordion'),
  'description' => t('Make pane or region accordion themed'),
  'render pane' => 'MYMODULE_accordion_style_render_pane',
  'pane settings form' => 'MYMODULE_accordion_settings_form',
  'render region' => 'MYMODULE_accordion_style_render_region',
  'settings form' => 'MYMODULE_accordion_settings_form',
  'category' => t('MYMODULE'),
);

/**
 * Renders the pane content.
 */
function theme_MYMODULE_accordion_style_render_pane($vars) {
  $content = $vars['content'];
  if (!empty($content)) {
    return theme('collapsible_text', ['title' => $settings['accordion_title'], 'body' => $content, 'anchor' => '');
  }
}

/**
 * Accordion settings form.
 */
function MYMODULE_accordion_settings_form($style_settings) {
  $form = array();
  $form['accordion_title'] = array(
    '#type' => 'text',
    '#title' => t('Enter accordion title'),
    '#default_value' => (isset($style_settings['accordion_title'])) ? $style_settings['accordion_title'] : '',
  );

  return $form;
}

/**
 * Renders the region content.
 */
function theme_MYMODULE_accordion_style_render_region($vars) {
  $settings = $vars['settings'];
  $panes = $vars['panes'];
  if (!empty($vars['panes'])) {
    return theme('collapsible_text', ['title' => $settings['accordion_title'], 'body' => $panes, 'anchor' => '');
  }
}

MYMODULE.module

/**
 * Implements hook_ctools_plugin_directory().
 */
function MYMODULE_ctools_plugin_directory($module, $plugin) {
  if ($module === 'panels' || $module === 'ctools' && !empty($plugin)) {
    return "plugins/{$plugin}";
  }
}