I think a lot of coolers used to have some parts like lid or even sides that were simply double-walled with an air gap and no real insulation. I remember old in

I think a lot of coolers used to have some parts like lid or even sides that were simply double-walled with an air gap and no real insulation. I remember old internet posts about people enhancing theirs by drilling into these spaces and injecting them with a minimally expanding foam from the hardware store.