With most modern-day Canon and Nikon flashes, it is possible to put a truly gob-smacking diffuser on the flash head the dwarfs the camera (like those offered by Gary Fong) — oddball accessories that conveniently hide you, acts as a windfoil, attracts the anti-terrorism Police and draws stickybeaks like bees to a honeypot. To say nothing of unbalancing already challenging deadweights. The StoFen is quite reasonable, simple, unassuming and unobtrusive, and will not obstruct the TTL flash sensor while so many others do. But it works well too with indoor bounce with added exposure (to overcome light lost to bouncing).

If you want soft light and have just one flash and a StoFen, prey for hazy to overcast weather. This will give you diffuse illumination across the scene with very light to no shadow. A fine day — especially between the high-noon summer hours of 12 to 3pm, will create huge swathes of shadows, massive variations in contrast and headaches with lighting and I'd generally look at packing two flashes and a reflector (held by somebody there to help you wrangle all this caboodle). Maybe start practicing with some willing subjects to refine technique before the day arrives.