Select an endpoint for your region

My solution is quite simple and it doesn't come with a fancy select list or anything like that for the GUI. Instead you just paste in the endpoint of your bucket or region and voilà—it should work.

Instead of setting the value property:

  $form['host']['#type'] = 'value';
  $form['host']['#value'] = 's3.amazonaws.com';

I'm setting the default_value property:

  $form['host']['#title'] = 'Endpoint';
  $form['host']['#default_value'] =  $this->get_host();
  $form['host']['#description'] = 'Select an endpoint for your region (e.g., s3.amazonaws.com, s3-eu-west-1.amazonaws.com).';

Here's the function that is being called to get the default_value property:

  function get_host() {
    $parts = explode('/', @$this->dest_url['host']);
    return $parts[0];
  }

To find the endpoint for the region your bucket is located in: http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region