debugProjectContainer.php

<?php

// short cached container instance

class debugProjectContainer extends Container
{

    public function getDefaultDocumentManager()
    {
        return $this->getDocumentManagerFactory()->get('default');
    }

}

DocumentManagerFactory.php

<?php

class DocumentManagerFactory
{
    private $class;
    private $options;

    public function __construct($class, $options)
    {
        $this->class   = $class;
        $this->options = array();
    }

    public function get($name)
    {
        if (!isset($this->instances[$name]) {
            $this->instances[$name] = new $class($this->options[$name]);
        }

        return $this->instances[$name];
    }

}

services.xml

<service id="document_manager_factory" class="DocumentManagerFactory">
    <argument>%document_manager.class%</argument>
    <argument type="collection">
        <argument key="default">%default_document_manager.options%</argument>
    </argument>
</service>

<!-- dynamically added document manager definitions -->
<service id="default_document_manager" class="%document_manager.class%" factory-service="document_manager_factory" factory-method="get" scope="prototype" synthetic="true">
    <argument>default</argument>
</service>