I'm not sure why you'd want to wrap your DSi in a thin jacket of metal. But if you did this would certainly be an option.
These NexiLux cases allow you to access all of the DSi's ports and don't increase the volume of the case noticeably. Of course they also let you change the colour of your DSi. Maybe if you could get the cases with art work or patterns on them they'd be more appealing to me.