Just got 4 of the cardboard tray free of charge from Costco. Quite a nice fit in the trunk. The remaining space is just right for a cooler.
Hope you don't live in a humid area, cause those boxed will sponge the moisture and then start to make you car stink like wet cardboard. (A) for ingenuity tho.
Okay thank for the advice, I will keep an eye on them. Or I can get new boxes from Costco every few weeks or so.