I have been using a bluetooth remote thermometer to keep track of the temp in the back of the truck under my GFC this summer I have two dogs that I want to keep happy and healthy.
My current solution is that I have been closing the latches on the rear side panel and then closing the panel and latching the front latch which leaves a ventilation gap. With running that set-up what I have found is that when driving around at 45+ MPH the temperature has rarely been more than +5 degrees F compared to what the ambient outside temperature has been. This is almost identical to what I saw with my leer topper that had sliding windows on the sides.
With being in Northern California and occasional triple digit temps what I have gone with is getting my dogs the Ruffwear Swamp Cooler jacket to help with those really hot days. Swamp Cooler™ Dog Cooling Vest | Ruffwear