Plywood would be a good idea. You could also use sheet aluminum, like the kind used for roofing or carports, you might even be able to get it in a matching cor. One other idea is to use lattice - not as peek-proof as the other two options, but would allow for better airflow under the TT, which could be a good thing.
2012 Jayco 32TSBH
2004 Ford Excursion
ProPride 3P / Tru Control brake controller
Roadmaster Active Suspension