The best solution I ever saw for this was this one friend of mine who built his softboxes out of 1 inch PVC tubing. With about $50 you can buy the tubing and the joints and make a box in 1/2 day. For the diffusion he went to a fabrics shop and got himself some of that material they use for parachutes, I dont know the name of it, he bought the lenght necessary for the frame he made and then put velcro to wrap around the tubing. If you cut a little 1 and 1/2 inch square out of each corner you can place the velcro and wrap it around the tubing. Once he was finished his stuff looked almost factory made.