Readonly ModelChoiceField of type in model foreignkey

[nelson fernando garcia gomez]

Hola estoy tratando en mi formulario (formsModel) implementar el atributo 
de *readonly*  a una foreignkey (ModelChoiceField)
pero no es posible alguna ayuda por favor GRACIAS así lo estoy haciendo: 
class CostoForm(forms.ModelForm):

"""
Edicion de Costos con formset.
"""
class Meta:
    model = Costo
    exclude = ('user', )

def __init__(self, *args, **kwargs):
    super(CostoForm, self).__init__(*args, **kwargs)
    self.helper = FormHelper()
    self.helper.form_tag = False
    self.helper.form_action = ''
    self.helper.form_class = 'form-inline'
    self.helper.template = 'crispy_template/table_inline_formset.html'
    self.helper.layout = Layout(
        Field('DELETE', css_class='btn btn-primary')
    )
    self.helper.add_input(Submit('submit', _('Aceptar')))
    self.fields['destination'].widget.attrs['readonly'] = True



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