I once oversaw some "heat aging" tests on Portra's immediate predecessor, VPSIII. Basically, the film spent time in a "hot box" at 140 deg F, then was tested at various intervals by exposing a sensi wedge, which was processed and evaluated.
As I recall, the VPSIII showed no changes at all, until somewhere around 300-400 hours, at which time it started going downhill at a steady rate. This behaviour was nothing like you would expect based on Internet lore.
My guess is that Portra film would have similar (or better) characteristics, such that 10 hours or so, possibly reaching 100 or 110 deg F in your trunk would have a negligible effect on the film.
Rcently processed the film from a Kodak disposable camera (400ASA) which had been in the bottom of a car luggage compartment for about 2 years in all temperatures. The first half of the film (shot about 10 years ago!) was poor, the second half, shot last autumn was as good as fresh film! (And the process by date was Sept 2005!)
I left some Kodak BW400CN in my glove box for a couple years, this was in Utah where it gets over 100F. It came out okay except it had some pastel pink and green splotches when printed on color paper. It was actually kind of pretty. Didn't notice an effective loss in speed or a change in contrast, but then I don't really shoot that stuff very often.