Hi, Keeping the box full seems to be very important,as mentioned earlier. Adding permanent insulation is very difficult & expensive in most cases. I''ve tried something very simple with excellent results.
I filled zip-lock bags with foam shipping peanuts. I fill in empty areas of the box with these bags.I have cold plates & I get a minimum of 24 hours between charges,with 30-36 hours not uncommon. This is in hot weather too. A little inconvenient,but very effective & cheap