For my 35mm I literally take it everywhere with me, or keep it at home. I have a Tim-buk-2 messenger bag, and there's a replacement Domke (IIRC) insert that happens to fit these bags perfectly, with room to spare.

So I can take 3 lenses, my camera and all filters with me and it's not much to carry honestly.

I'd never like to leave film in a car, though the condensation wouldn't be a problem if you put it in an zip-lock... same with cameras probably. A little fogging maybe, but if it's hot enough to be a problem, that condensation will evaporate pretty quick once out of the cooler.