If you want to reinvent the wheel then try coro.
Steal ideas from places like
http://www.airflowsigns.com/page9.html
Don't want to invest in hardware? Try attaching the mags to the coro with VHB tape as they will only be used for temp applications and it will last. That stuff is super-strong and will hold a fender to a car. The coro will rip apart before the tape gives out.
If you were an ASI/PPAI member you could get free or low-cost samples then use what you get to engineer your own.
A must is covering the mags with something (cheap vinyl) to keep the car roofs from being scratched.
Be sure to have a disclaimer sheet (signed by the client) whereas you do not warrant the signs as once they leave the shop you don't have control over installation and use and that they assume full responsibility for end-use.
In any case, whatever you engineer, you had better test well.
Oh, and using neo-magnets (neodymium) is the only way to go.