Going to build my own soft-box. But what materials do people commonly use? A thick sheet of nylon? What materials are used in the pro kits? I plan on using fluorescent lights.
With diffusion you need to be concerned about heat resistance. You need something that won't be setting off overhead sprinklers and causing a big fire; it happens.
Go to the ROSCO web site (below) and find their Cinegel. Look for 3062 Silent Grid cloth and order what you need. It's the cheapest and most direct route to getting what you want at a fairly low cost. While your at it ask for a sample swatch book of all their gels.