Just pop by a local sign shop and have them match the "EDITION" type from the side IronMan decals in a good quality UV-resistant vehicular outdoor material like vinyl or scotchcal. They'll make it up whatever size you want, and if you go for the look and proportions as on the side decals it'll look like it came that way.

Last time I had something done up about the size you're looking for it ran about $25.