I have found a company making some very nice plastic stickers of airplane outlines. Many of them are WWI and can be found here in WoW-- some are nice planes we don't have here. (they also have WWII and jets and other things.)
These are "outdoor grade vinyl" and could work on autos if you want. They work very nicely on other places.
Right now, they only have an "ebay store."
If that link does not work, just search for Flightline Fabrications on ebay.
Nice people to work with but just getting started in the business.
Mention "kduke" when you write them and it will smooth things for you to get a "volume discount" for buying several, because you will certainly want more than one or two of them (and help me a little bit.)