I had one of these made for my restored 914 from a tracing I made of the hood. I'm sure they'd do one for the fortwo.
http://www.magnetbra.com/
Oh, woops, the smart is plastic...it won't work.
I'm sure if you google "car bra" one of the big manufacturers will make one for you if it doesn't already.
I'd just apply Rejex and carry a bottle of detail spray and a towel. They'll come off easily without having to wash the whole car. You'll have to wash the bra anyway-bugspots on black vinyl will look pretty awful in short order.