You can certainly build a box within your closet--that is what I did. You will need seperate doors on this box however, as a closet door will never seal out 100% of the light. I am heading back to Idaho sometime tomorrow. I will try and remember to snap a pic of my closet. I made it out of 1/2" plywood and painted the interior with flat white paint. I used this as my vegging space for many years and also did a small scrog grow once that I ran all the way from clone through flowering.
You will not want to vent directly into the attic though unless you have a whole house attic fan up there somewhere to exhaust that hot moist air out. Exhausting directly into the attic and not exhausting it out of the attic can cause mold.
By the time I got fittings on the ends of the cool tube, I found that I could barely fit it into a 32" space. I ended up putting it in diagonally so it would fit and I could get ducting on the tube. I don't think I could have ever gotten a carbon fer in there. You are correct in that a passive inlet and active exhaust is the best--this is what creates the negative pressure. How hot the space gets will depend on the fan you use and the temps of the incoming air. If the incoming air is not cool enough, it will not cool your space. I always pulled air from my crawl space, which stays cooler in the summer and warmer in the winter. I also run my flowering lights at night. This helps with heat issues in the summer and cold issues in the winter.
And no matter how well you believe that you plan your space, you will be making changes to it as you gain more knowledge and experience. Also as we get better products and methods of growing, you will want to try this or that. So expect that and embrace it. A grow room is not a static thing.
