I hang a light colored cap over the pods of any detectors I carry in my truck mostly so they don't bump each other or something else and get marred. I always have several detectors in the back seat... never had a bit of a problem either but I drive and extended cab F-150 with heavily tinted windows in the back part. You can see in but not easily, you have to cup your hands to the glass to get an idea of what is inside and it still isn't clear. I can see out just fine though. The air gets hot in there but the detectors really don't get as hot as you'd think. They are never hot to the touch, never as hot as they may quickly get out hunting in the sun, especially when you lay it aside to recover a target. Black absorbs heat and detectors have been black forever... of course we haven't had these LCD displays for all that long. An anti-glare screen protector may help to deflect some of the heat too, but i'm not sure. I still think using a pod cover in the sun probably holds extra heat... sort of like wearing a black plastic coat while you are detecting in the sun... try that and see if you get hotter.
Julien