How to fill a big softbox with hot-shoe gun.
I've found using the wide panel works pretty well, better than the other trick of a Stofen-type diffuser cap, though YMMV. But for really big ones, say bigger than 100cm, try a modified Stofen.
Start with a double-diffuser softbox. You need to get more light out to the edges, so take the Stofen (or similar, dead cheap on Amazon) and drill or cut some holes in the sides to force more light out in that direction. You may need more, so take some tiny squares of kitchen foil, size of small finger nail, and stick them inside the front.
You'll need to fiddle about a bit to get the best balance. It works, but you'll be pushing the gun for power. There comes a point when hot-shoe guns are not really up to it.