Hi,

You can inherit the view as following:

<odoo>

    <data>

        <template id="my_digest_module.digest_mail_main" inherit_id="digest.digest_mail_main">

            <xpath expr="//div[@class='o_mail_summary']" position="after">

                <!-- Add your custom HTML here -->

            </xpath>

        </template>

    </data>

</odoo>

Regards