cover.xml

<?xml version="1.0" ?>
<model xmlns="http://namespaces.plone.org/supermodel/schema"
       xmlns:form="http://namespaces.plone.org/supermodel/form"
       xmlns:i18n="http://xml.zope.org/namespaces/i18n"
       i18n:domain="collective.cover">

    <schema>

        <field name="template_layout" type="zope.schema.Choice"
               form:widget="collective.cover.widgets.selectpreview.SelectFieldWidget"
               form:omitted="z3c.form.interfaces.IEditForm:true">
            <description i18n:translate=""
              >Choose one of the predefined layouts</description>
            <title i18n:translate="">Layout</title>
            <vocabulary>collective.cover.AvailableLayouts</vocabulary>
            <default>Empty layout</default>
        </field>

        <field name="cover_layout" type="zope.schema.Text"
               form:omitted="z3c.form.interfaces.IForm:true
                             z3c.form.interfaces.IAddForm:true
                             z3c.form.interfaces.IEditForm:true">
            <description i18n:translate=""
              >The layout to be used to render groups and tiles</description>
            <title i18n:translate="">Cover Layout</title>
        </field>

    </schema>

</model>