I keep 6-7 film holders in a backpack with my camera, and about the same number in an insulated film bag. On "day trips", the film bag stays home, but I do take it along if we are going to be away for several days.
I worry about damaging the film by leaving it in a closed car parked in the sun. If I have to leave either the backpack or the film bag in the car, I try to leave the windows cracked open and make sure that neither bag is actually in the sun. I drive a Subaru Forrester and have a cover for the rear storage compartment that protects the contents from both direct sun and curious eyes. And I try to park the car in the shade.
I recall hearing a LF photographer (either Tillman Crane or Michael Smith - memory is not perfece) say that he doesn't worry about short exposures to high temperatures.
The film bag is actually an insulated picnic bag. In addition to spare film holders, I have a box of unexposed film, and three boxes (N, N- and N+) for exposed film. If I encounter an N-2 situation, I will put the exposed film into a plastic envelop before putting it in the N- box.
When we travel we stay in either hotels or time-share condos, and usually it is possible to go into the bathroom, roll up the bath mat to block the bottom of the door, and achieve complete darkness for loading holders. But I also take along a changing bag (wrapped up in a plastic bag to protect it from dust) in case I have to "rough it".
The film bag also includes a brush for cleaning holders, spare batteries for the light meter, a couple of rolls of 35mm film and a small 35mm rangefinder camera, a Polaroid 545 back, a few "wash and dry" hand cleaners, a Sanford Sharpie pen, my PDA and cell phone, and a box of Lenscrafters lens wipes (for my eyeglasses).