need to set the length of the whole PDU manually
        to avoid any bit fiddling use a dummy class to build the layer content

        also add padding if frame is < 64 bytes

        Note: padding only handles Ether/n*Dot1Q/EtherCat
              (no special mumbo jumbo)

        :param pkt: raw string containing the current layer
        :param pay: raw string containing the payload
        :return: <new current layer> + payload

def post_build(self, pkt, pay):
        """
        need to set the length of the whole PDU manually
        to avoid any bit fiddling use a dummy class to build the layer content

        also add padding if frame is < 64 bytes

        Note: padding only handles Ether/n*Dot1Q/EtherCat
              (no special mumbo jumbo)

        :param pkt: raw string containing the current layer
        :param pay: raw string containing the payload
        :return: <new current layer> + payload
        """

        class _EtherCatLengthCalc(Packet):
            """
            dummy class used to generate str representation easily
            """
            fields_desc = [
                LEBitField('length', None, 11),
                LEBitField('_reserved', 0, 1),
                LEBitField('type', 0, 4),
            ]

        payload_len = len(pay)

        # length field is 11 bit
        if payload_len > 2047:
            raise ValueError('payload size {} exceeds maximum length {} '
                             'of EtherCat message.'.format(payload_len, 2047))

        self.length = payload_len

        vlan_headers_total_size = 0
        upper_layer = self.underlayer

        # add size occupied by VLAN tags
        while upper_layer and isinstance(upper_layer, Dot1Q):
            vlan_headers_total_size += 4
            upper_layer = upper_layer.underlayer

        if not isinstance(upper_layer, Ether):
            raise Exception('missing Ether layer')

        pad_len = EtherCat.ETHER_FRAME_MIN_LEN - (EtherCat.ETHER_HEADER_LEN +
                                                  vlan_headers_total_size +
                                                  EtherCat.ETHERCAT_HEADER_LEN +  # noqa: E501
                                                  payload_len +
                                                  EtherCat.ETHER_FSC_LEN)

        if pad_len > 0:
            pad = Padding()
            pad.load = b'\x00' * pad_len

            return raw(_EtherCatLengthCalc(length=self.length,
                                           type=self.type)) + pay + raw(pad)
        return raw(_EtherCatLengthCalc(length=self.length,
                                       type=self.type)) + pay