I played around with a similar design to add to my MAK. Still have the plans and a place to have it made, but I'm not sure it is the best way to go, at least for me. Basically it allows you to hang sausage in the cold smoker side. As delivered, the space in the MAK is limited for cold smoking sausage. You can't hang sausage.
From the prototype I made quite a while back, the taller cabinet filled nicely with "cold smoke". I found that I lost some of the smoke due to the side vents on the original box. In my redesign I had to block those off and provide a vent toward the top to "pull" the smoke through the new box. It might be cheaper to buy a Bradley or find a proofer and adapt a Traeger type fire pot, kind of like Rick has already done. That is where I am right now. I'm in no hurry to make that decision. It only costs money.
Art