Ours has about a 1 deg. difference. It's a 6'-4" tall freezer. We have 2 computer fans on the 2nd shelf from the bottom. They blow down onto the heat lamp and tile we have set up in there. Each shelf has a few 2 litre bottles of water, for mass to keep temps stable.

I would move your fan to the middle at least, blowing down.

Do your fans stay on constantly? Or do they only come on with the heat via the t-stat? Ours are constant on.