+1 for Aerogel, Cryogel.
We did our box several years ago, also made a portable "freezer" for meat and ice cream by trading a pair of these out of our
freezer compartment each day:
https://www.amazon.com/Arctic-Ice-Tu...tic+ice+tundra
and a separate "refrigerator" for veggies using a pair of these:
https://www.amazon.com/Arctic-Ice-Al...tic+ice+tundra
The refrigerator runs a lot more because we have more than doubled the effective size of our box but with plenty of
solar, who cares?
My wife is an accomplished seamstress and made a couple of cryogel insulated zippered bags which live under the forward berth. They fit the space perfectly and were made to fit the largest size
freezer brick/refrigerator brick.
Our freezer compartment JUST FITS one freezer and one refrigerator brick.
Each day we swap the pairs.