Sorry about bumping the old post. I don't do the vinyls personally but I've designed one of my own and could do more. They are far from great I know but there's no choices when it comes to boxies.
I could modify my design to fit other's interest or I could do one based off of anyone's car. That design could probably be taken to any decal shop and done for pretty cheap. There's even a website that does it from files, I have no experience with that but here's the link.
http://customvinyldecals.com/CustomDecals.htmlThe only thing is if I do a custom decal don't expect it overnight, it would take me a while. Not that they are hard to render but 20 college engineering credits, lacrosse and working keeps me really busy. And most of my free time is spend on my wagon. I'd happily do one for free but expect atleast a week and a half.
Here's one I made, same as my sig.
Let me know if I could help you.