Personally, even in a car, I would assemble at the destination once ready to eat. You can do it if you have separate containers (or foil/bags) for the components, and napkins/kitchen towels to catch the mess. If you must assemble before the 1 hour drive, at least skip the lettuce or add that last minute? I say skip the lettuce entirely and use cilantro (and chopped onions) instead if possible. This might be too late and if so, hope you had a lovely time. I'm sure she appreciated your efforts no matter what.
