Getting professional assistance in the design phase is good, but I didn't have that available, so I winged it. Put in 4"+ of rigid foam insulation
(couldn't get polyurethane
, so used fine-grained blue Styrofoam). Bought an evaporator that's wrapped in a box, put a bottom on it, & mounted it vertically, then wrapped it in a thin layer of insulation
. What's inside the evaporator freezes solid, & the rest of the box stays cold but doesn't freeze the lettuce. That was 17 years ago, & the system is still working well (although I had to replace the evaporator last year). It makes for a very small freezer, so it generally only gets boneless meat, but it holds ~20 pounds.