I found a cheap 3" foam pad that fits perfectly in the 4runner at home depot for $25. The measurements are 72"x24"x3". I cut it in half and it fits in the trunk with the seats folded up when not needed. I haven't slept in it overnight yet but initial tests seems very comfortable.
Future Foam 3 in. Thick Multi-Purpose Foam-10030BULK3 - The Home Depot