Just thinking aloud ... by no means I am correct, or that my opinions are the most effective. Different togs will use slightly different techniques to try and achieve the same result.
Assuming you want a diffused light source, but still want to control light spill.
Control light spill - can use grids and flags. With your strobist set-up, I suspect flags might be the simplest.
12" x 16" softbox sounds too small to light an entire person evenly (assuming your models at least 1.68m and taller). It might be ok for a head and shoulder MCU with the soft-box fairly close and use small reflector for fill. If you get hot-spots with that small box, just attach a layer or two of tracing paper over the center. When using a small softbox from a further distance, the quality of light resembles a slightly diffused
point source.Can try using what you already have and fire that softbox into a reflector or better, through a large diffusion panel. You have to determine the box to reflector/diff panel distance that works best. Light spill will be a problem. Use large, cheap boards as flags.
Another possible method is to mask off the sides of the reflector or diff panel with black paper to sort of give you an 'aperture' from/through which light is reflected/shot through. Adjust the angle of the panel to control where you want the light to spill. Depending on your specific shooting situation, you might have to (ironically) flag-off the sides of the small softbox as well to minimise light spill. Might be easier to control if you DIY barn-doors out of light-weight board.
Note: I've never tried the last method, so I can't say for sure whether it would work, or how effectively it would work ... or not. 
If you already have like 2 or 3 speedlights, you could line-up them up with one soft-box each and form a small bank. But to control light spill, you will have to DIY grids for the front of the soft-boxes, unless that particular model has one designed for it ... or use flags.
Perhaps lighting experts like DP and Ukay can offer better options cos I'm just thinking off the top of my head. Would love to find out how others would approach the same situation, given your constraints (small soft-box and I'm guessing - you want extreme portability and to carry as little as possible on shoots)
Of course, the easiest is to get the most appropriate equipment for the task at hand.
