Why don't you modify the existing footer template or create a second one which you only use in this report?
You can do this under report/views/layouts.xml. Here is the template for the default footer and header.
You have four options:

1. Modify existing footer (addons/report/views/layouts.xml)
2. Create a new footer template (addons/report/views/layouts.xml)
3. Add the code directly in your report.
4. Use the default HTML field for a custom footer under your company settings.

Default footer:

<template id="invoice_layout_footer">
    <div class="footer">
        <div class="text-center" style="border-top: 1px solid black;">
            <ul t-if="not company.custom_footer" class="list-inline">
                <li t-if="company.phone">Phone: <span t-field="company.phone"/></li>

                <li t-if="company.fax and company.phone">&amp;bull;</li>
                <li t-if="company.fax">Fax: <span t-field="company.fax"/></li>

                <li t-if="company.email">&amp;bull;</li>
                <li t-if="company.email">Email: <span t-field="company.email"/></li>

                <li t-if="company.website">&amp;bull;</li>
                <li t-if="company.website">Website: <span t-field="company.website"/></li>
            </ul>

            <t t-if="company.custom_footer">
                <span t-raw="company.rml_footer"/>
            </t>

            <ul class="list-inline">
                <li>Page:</li>
                <li><span class="page"/></li>
                <li>/</li>
                <li><span class="topage"/></li>
            </ul>
        </div>
    </div>      
</template>

You could choose to modify this one or you could create a second one. For example:

<template id="external_layout_footer">
    <div class="footer">
        <div class="text-center" style="border-top: 1px solid black;">
            <ul t-if="not company.custom_footer" class="list-inline">
                <li t-if="company.phone">E-mail: <span t-field="company.email"/></li>

            </ul>
        </div>
    </div>      
</template>

You could then call report.external_layout_footer in your report. Or you could directly add this code in your current invoice.