I wonder which is a better, cheaper insulator for refrigeration systems? Anyone have any idea?

Frankly I am looking to upgrade my "ice box" with something better, but don't want to tear the whole galley apart to do this. I don't mind losing a bit of volume in the box.

I have an older Alder Barbour system, but I don't think it is working... so I want to upgrade everything, but would rather not tear apart the whole galley doing this.