Seems like we may as well haul a mobile garage to protect Sienna!!I'm reviving an old post. Just found out about an interesting product that may help with the CAT theft issue, not sure how well they work for the Sienna but it may be helpful for your other vehicles.
Most Effective Product to Protect against Catalytic Converter Theft. Catstrap Provides a Physical Barrier that is extremely difficult to cut using traditional cutting blades. Cateye Provides a specialized motion sensor that activates a 130 db siren when the thief gets close to your converterwww.catstrap.net