Rather than create a control out of curves from scratch, I decided to import on that I made earlier.
It was simply a linear NURBS cube, which is ideal for creating controls as it does not render. I snapped the cube to the chest joint and then scaled it out.
Next were the abs control and hip sway, the latter of which would sit within the hip control. These were both created from NURBS circles. The Hip sway was shaped simply by going into Vertex Mode, selecting every other vertice and scaling in.
Finally, I constrained all these controls to the relevant clusters using a Point orient. The root joint was constrained to the Hip control using a Parent constraint, as I wanted it to stay in its own place, but still follow the Hip control if need be. This is a video to illustrate me constraining all the relevant joints and clusters and then testing the controls. I created a layer called DO_NOT_TOUCH, for anything that I do not want the animators to mess with.
I then parented all of the controls to the Hip control, meaning that if I dragged the Hip control around, the rest of the upper body follows.