I'd probably score it with a carpet/utility knife or if available to you - mark it with a good visible marker and use a Dremel with a reinforced cut off wheel. Make sure you wear a respirator and goggles though - that dust will get everywhere and the fumes from the acrylic as it cuts/burns will stink. I would suggest using a 2X4 and using that to clamp it down onto a work table or sheet of plywood to keep the diffuser from "kicking up" as you cut it (it shouldn't - but if it does it will crack and that'll be the end of that piece).