It's always fun watching a new build!
Anyway, if my calculations are correct heres how I would do it!!!
Chamber volume-- 75.39 cu ft.
box needs to be 1/3 of that which is 25.13 cu ft.
The box I would build would be 3X3X3, which yeilds 27 cu ft.
Once you build the rack in it to let the ash fall through you would be just slightly over 25 cu ft.
Hopes this helps. I would recomend insulating the box, the cost for the insulation is around $40 plus the metal for the outer box but it really help with heat control and it uses alot less wood.
Its Si-intistic, Jack!
One lovely UDS
Shanaynay--Custom Home Built Smoker on Wheels