Interesting project. Try completely blocking the vents in the freezer and see what temperature you get. If it's still too cold, cover the back panel with spray foam or fiberglass insulation. If it's too warm, uncover a little bit of vent at a time until you get the temperature you want. You'll want to uncover the same amount of incoming and return vent. You could also cut holes in the divider but that might cause bad airflow and temperature variation. Not to mention, you might cut through a refrigeration line.