Best practice for providing an Add Payment form for offline payments

I have been reading through the issue queue, the code and documentation and I'm quite confused about adding a payment form, either custom or standalone.

I am looking to add a form to allow admins to account for payments received offline, unassociated to specific purchasable entities. Is this possible?

To summarise, I looked at "Writing a Context" documentation.

To create a payment form, it's easiest to create your own custom form and use payment_form_embedded() directly to get the core payment form elements, or to display payment_form_standalone() by calling drupal_get_form('payment_form_standalone'), which calls payment_form_embedded() for you.

I created a module with the following:

 /**
 * Implements hook_menu.
 */
function payment_management_menu() {
  $items['admin/config/services/payment/add'] = array(
    'title' => 'Add Payment',
    'description' => 'Form to add a custom payment',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('payment_management_payment_add'),
    'access arguments' => array('administer payment configuration'),
  );

  return $items;
}

/**
 * Custom form for adding payments.
 */
function payment_management_payment_add() {
  return drupal_get_form('payment_form_standalone');
}

I get the error:

Recoverable fatal error: Argument 3 passed to payment_form_standalone() must be an instance of Payment, none given in payment_form_standalone()

I have tried the "Add a new Payment" field method, but I would prefer to have a stand alone form untied to an entity via a payment field. I also cannot edit the "Account" value using this method, ideally this would be editable on the form I want to present the admins.

There are multiple issues relating to this, which I added as related issues, but I cannot find a desirable solution or currently recommended method for this presumably common use case. If this exists in the documentation as stated in #2165269-5: Provide "add" form for payments, could you please provide a direct link to the part of the documentation, or an example of this in action.

Thanks in advance.