Keeping produce fresh with refrigeration methods that require no external power.
First I must apologise for not replying to your enquiry before. I have had computer problems and for some reason this one seems to have slipped through the net.
You are absolutely correct in that evaporative coolers work best in areas of low humidity. I would think 65 to 75% RH should provide some cooling perhaps 5C or so. Air flow is also very important, try and place the cooler in a place with wind. This will move the moisture away and assist cooling. It is also important to get rid of field heat as quickly as possible so a bath or spray of cold potable water before cooling will help.
Many years ago ITDG reviewed evaporative coolers in India and there is an article in Food Chain. If you would like a copy of the article please email firstname.lastname@example.org.
I can only suggest you build a small, low cost unit and test it to see what happens. A small reduction in temperature can have a profound effect on shelf life.