Actually for the low cost alternative foamcore can be made into a lightweight and fairly durable softbox. Cut this into the proper trapezoidal shape for the back and sides and tape together with duct tape then tape a piece of cloth over the front and you're in business. This can be modified with a foil reflector on the inside, or for more spread also put a circle of foil on the back of the cloth facing the flash to bounce the light back to the softbox sides.
I also made a large diffuser with a bedsheet and steel conduit parts from Home Depot that stands on its own for about $30. It's about 6x7 feet.