The alternative to the GUI "custom box" is not to edit the config.xml, but to configure unbound directly from CLI (i.e via serial console) iiuc. ;-)
server:include: /var/unbound/steven.confinclude: /var/unbound/custom.conf
server: domain-insecure: "XXX" domain-insecure: "YYY" local-zone: "YYYY." nodefaultremote-control: control-enable: yes control-interface: xxx...