Haven't been here in months, so late to this thread.
Make a PVC frame in whatever size you need. JoAnn Fabrics (online and local) and others sell rip-stop white nylon in 60 inch widths at about $7 per yard, which is what many of the soft boxes use, as JD mentions. It's what I use. I sometimes double for even softer light. You can vary the effects a lot with a rip-stop/PVC panel by moving the light source closer or further from the panel, making the effective light source larger and softer, or smaller and harder.
You could also get Rosco diffusion materials in 20x24 inch sizes for under $6. I mount these with velcro in a frame made from a science project 3 panel foamcore background, with the side panels acting as gobos (blocking cards) for the light source. Rosco makes a number of diffusion materials, with sample photos shown on their website.
Another option often used in studios is to tape two pieces of foamcore together along one edge. Set this "V" panel up on end and point the light into the V so that it bounces back out onto the subject for nice soft, even light.
If you want a smaller diffuser panel that's not square, you can stretch rip-stop nylon on a round or oval embroidery or quilting hoop and use that in front of the light.
Lee