Apple went through this phase where they would use a white lined acrylic for all there products. It was great for letting light through, which they even used to that effect with the status lights in the iMacs of the time.
I have made a lot of flash diffusers in the past, using anything I could get my hands on, from old milk bottles to marscapone pots. They all do a great job, but they don't last 2 minutes when out in the field. I saw this old Apple Airport laying around at Nottingham Hackspace
and it immediately hit me that it would make a great solid long lasting flash diffuser.
The results are great, heres how to make your own.