77W is right. If you're going to ventilate a rack, that means you're going to create a pathway for air to go into the rack and go out of the rack and provide a means to make the air do that. For ventilation that you do on purpose, you need closed sides and back and openings specifically placed for air to come in or go out.

Let's assume you'll have sides. OT, but: sides, because of their cost, indeed make the Slim5 not a great choice. Moving on... you were going to force the air out of the top of the rack. Do it just as you would with a foot of ceiling space; just be aware that there will be a bit of resistance to air flow.

I think I have a much cooler idea, mostly because it's silly and outrageous yet will work. Do your purposeful forced ventilation as you had planned. Open up the ceiling above the rack. Make and install a sealed duct between the rack's upper opening and the ceiling opening. Some shiny aluminum would be just the corny Robbie-the-Robot touch. Somewhere else a couple of feet away, along the same joist space, open up the ceiling to let the air escape. Increase your forced ventilation to overcome the resistance this route offers. Insulation between the joists is a difficulty, but if the cellar is going to be kept warm for TV watching, there's little reason for heat insulation in its ceiling.