> Would I be better off taking care of the acoustics as a part of the construction (built in) or use modular panels such as “Real Traps” and/or homemade traps after the walls are built? <
I don't know how you'll find room to build treatment into the walls with only 9 x 12 x 8 feet to work with. Better is to plan on having traps straddling as many corners as you can manage. The ceiling corners are great because they're up and out of the way. But you'll do best if you put some traps across the regular wall-wall corners too.