3" is not that thick. In Australia there is a company that makes portable fridges for 4wd etc with 125mm (5 inch) thick walls. It is 24% more efficient than 75mm (3 inches) thick wall fridges. Most people don't have the room in theirs cars, so use cheaper thinner walled ices boxes. Simply adding 25mm (1 inch) to a portable fridge which have very thin walls of about 40mm (1.5 inches) to start with, will reduce battery load by roughly 25%. If you are building a custom ice box chuck as much extruded polystyrene at it as you possibly can. Another option is Aerogel, but it is expensive and I believe more fragile. Aluminized material only works with a radiant heat source, eg the sun, hitting the ice box. So make the outside of the box silver or white. Keeping the ice box/fridge &/or car in the shade will be more beneficial, & not cost anything. I have had 65°C (149F) in the car without really trying in Australia. Your Esky as we call them (ice box) takes a real hammering. 75mm (3 inches) with care in the shade in summer with a solid block of well prepared ice will last 7 days. The solid block of ice should be 50% or the volume of the fridge. The ice box should also be pre-cooled for 24 hours. Also have an official "esky door opener" so the other person can get the items in & out of the fridge at quick as possible & you can provide a third hand! Opening the ice box is a 2 person task, teamwork to reduce opening time & number of times of the ice box door is of the utmost critical importance! Kids are not allowed to open ice boxes if you want/need to keep your ice as long as possible!