Here in the UK, the very best light diffuser for optical use (scanning) is from Perspex.
The material is a cast acrylic, code is 1TL2, range is LED Spectrum, and it passes 51% of the light (so loses just 1 stop) and has a flat curve across the visible spectrum (and beyond), so there will be zero color casts.
There are various sources to get this, some places will pre-cut to your req’d size, some will drill holes as you need.
If you are un the US, then there us a Plexiglas equivalent - check their website for LED/light diffusion material.
These diffusers are quite expensive (for diffusers) - you canb get far cheaper parts, but they will have a nasty colour case that is non-flat, so you’ll fight to remove the cast as it’s not even.
By “expensive”, I’m talking maybe <£20 for an A4 sheet… so not that expensive at all really, for a one-off.
Hope this helps