If you want to design a car while going single rider - and the design part is more fun for me than the ride - there's a couple of kiosks in the post-ride area where you can create the design, taking as long as you want, then don't do *any* design when you go through the line. When you reach the boarding area, touching your band will load the car you designed.
This, by the way, is one of the very few things where something is associated directly with the magic band itself, rather than with the associated MDX account. If you design different cars with different bands, you can keep them both separate. The design is associated with the RFID number itself. (You may be able to design cars using *anything* that's RFID, but I haven't tested that theory yet.)