reporting-engine/report_wkhtmltopdf_param/views/paperformat.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <!-- FORM: report.paperformat -->
    <record id="paperformat_view_form_custom" model="ir.ui.view">
        <field name="name">paperformat with custom parameters</field>
        <field name="model">report.paperformat</field>
        <field name="inherit_id" ref="base.paperformat_view_form" />
        <field name="arch" type="xml">
            <field name="report_ids" position="after">
                <field name="custom_params">
                    <tree editable="bottom">
                        <field name="name" />
                        <field name="value" />
                    </tree>
                </field>
            </field>
        </field>
    </record>
</odoo>