Well all I can say is that I have been shooting pictures (all formats) for over 40 years, and professionally for over 32 years, and I have never felt the need to take special measures to keep my film cool when traveling. Yes, a closed, parked automobile in the summer can get quite hot in the passenger compartment due to the heat gain thru the windows. Hint: Leave a window down a tiny crack helps prevent excessive heat build up. The trunk (boot..in UK) is a better place for surplus film. You do what you like, but carrying a dozen or 2 rolls of color and b/w film around should not be a problem for anyone. Like I said, film is comfortable for reasonable lengths of time (weeks) at any temperature you can tolerate. Long term storage is another matter.