Shutterstock has lovely video tutorials on YouTube. PetePixel has found another great one among them that suggests you to use a simple shower curtain for light diffusion.
Soft light depends mostly on the size of the lightsource and its distance from your model. This video compares the effect of a barebulb flash, a softbox, a diffusion panel especially designed for photography/cinematography use and a simple $5 shower curtain. Surprisingly the effect of the curtain is almost identical to the expensive Fotodiox diffuser.
Maybe you should check it out for yourself at home.
