My freezer that I am getting ready to enclose is the type that breath through the skin. I plan to leave 2" of space between the decorative wood enclosure and the skin. I also plan to leave the bottom completely open and about 2-3" (the height of the casters) off of the ground. The only exception on the bottom will be a single 2"x4" strip running through the center for strength. The back of mine will be completely open as well.
Like you, I have seriously been considering whether or not I need a fan(s) to move air around or if the 2" gap and passive cooling will be adequete. I am seriously leaning towards passive cooling at the moment. If I do go with the active (fan based) cooling it will just be a couple of muffine fans on the outside to draw cool air up through the bottom. Since it seems I am in the same boat as the OP, what are your opinions on leaving the back open and leaving the gap with no fan based air and the open bottom?