I had a similar problem on an old walk in box. The KISS solution was simply to utilize a mechanical thermostat with a remote sensor bulb and set the differential to about 5°. The thermostat was wired in series with the primary control thermostat, which was set to about 43°.

It's been a while however I think I ended up sitting the cut out at 32°. This was a very old installation that needed to last about another year until a new facility was finished. I previously tried to increase airflow across the evaporater which helped but didn't cure the problem. The mechanical thermostat set to a 5° Delta T worked like a charm, I never had a freeze up after the installation, and was able to maintain a consistently sub 45° box temperature.

PS. I located the secondary thermostat sensor bulb on lip of the coil, where I typically began to ice up.