So, for the diffuser we used Solacryl, which is a clear UV-transmissive acrylic (1/8" thick). We then use a hand-held sander to sand both sides of the material. After sanding, give the materail a rinse in water and this makes a pretty good UV-transmissive diffuser.
You can search online for Solacryl material. In my experience it is not that expensive, but you will have to buy a sheet and cut it to the size you need. We have bought our Solacryl sheets from a company called Loop Acrylics in the US. You can contact them by email at email@example.com. I hope that helps !