PyraSied is a local Dutch company that specializes in plastics and acrylics.  I recently saw a sample of their PyraLED PMMA (acrylic) diffusion sheet and was supremely impressed by its diffusion properties while maintaining decent transmission.  Specifically the PyraLED 3 in 10mm thickness.  It is the best diffusion I’ve yet seen from any supplier.

Thanks BeersNielsen for finding this material!