Also, think outside the softbox for diffusion options.

Stretch some translucent fabric on a frame and use it as a diffuser. I've seen really big ones made out of military surplus parachutes. In the movie business this is called a "silk."

Bounce the light off a sheet of white foamcore or foamcore covered with aluminum foil (or make it one side white, one side silver).

Bounce a light off a sheet of foamcore, and then put your fabric diffuser between the light and the subject, so the bounced light is coming back through the fabric for lots of diffusion--softer than a softbox and heat isn't as much of an issue, since it's an open structure.