I could not live without a potty in the trunk! My youngest has been trained for almost a year but still sometimes fails to give me much notice of her need to go. Plus, I have 2 kids in car seats and there is just no way we'd make it in to a potty if I already have them buckled in.
I don't use any special seat...just one we outgrew in the house that migrated to the trunk. We tend to use it in parking lots...like at the zoo or Sea World where the nearest bathroom is likely a hike away beyond the entrance gate (we have a minivan so I can close the doors for privacy and still have space for her to maneuver). So, dumping it in the weeds or bushes isn't an option. I keep a roll of paper towels and a stash of grocery bags with it. When she is done I stuff it full of towels to absorb, dump them into a bag, wipe it with a Lysol wipe, and tie up the bag for the next trash can we pass.
It's easy and in NO way replaces the big toilet or reinforces potty fears. For us, it's simply a necessary convenience (is that an oxymoron???). :)
Oh, and I have a tube of wet ones in one of my cup holders and usually have hand sanitizer in my purse.