With all due respect to Surapon, those little on-camera, postcard sized "softboxes" are totally useless.
If I understand, you want a real softbox, affordable and portable.
or smaller, a bit more rugged, but loses some light due to the locking assembly inside:http://www.bhphotovideo.com/c/product/925399-REG/westcott_2030_rapid_box_octa_mini.html
Check out some of Neil Van Niekerk's excellent blogs and videos showing how to get amazing results with these types of modifiers or even just bouncing the on-camera flash.
Really, forget about the "softbox" you can mount on-camera. The Rogue Bender mentioned already would be much, much better.