As Bart says - almost any kind of white translucent Plexiglass/Acryll will do.
The one thing to keep in mind is the thickness, I've tried 2.5mm and 5mm and kept the 5mm. The 2.5mm didn't seem as "uniform" (though that might have been a subjective feeling) - but the 5mm was also considerably darker.
However I'v found that it's just the perfect thickness for me as when I take the LCC photo, I'll simply turn the camera toward the light source of my scene and I'll get a perfect middle grey tone. Not too bright and not too dark, so Capture One has an easy time with the file.
As Bart suggested, it won't be neutral so you can't really use it for white balance.