In general, you are going to want your freezer, if it's of a decent size, to have 2 coolers blowing in to it. In the higher temp days (especially a heat wave), a single cooler will struggle to keep your food frozen. I like to use a 50 - 60 tile freezer (6 x 9, typically) with an external wall to blow the hot air out. I don't like the micro of building and tearing down roofs to min/max the direction of the hot air; I just blow it outside or in to a central chimney.
I personally like to keep my temps set to -2C to keep the room in a state where the food will stay frozen even if there is a lot of foot traffic opening/closing the door.
If you have animals in your colony, make sure you set up an Animal zone that excludes your freezer. Animals seem to love to hang out in the freezer, and all that extra opening of doors can overwhelm your coolers. As you get more and more animals tamed, they can end up having the door almost permanently opened.