This is in response to a question on Twitter, about making a light in the style of a bar graph, involving ten panels.
The least labor intensive & most even diffusion would be to use opaque acrylic sheet. (This may be cost effective if you are doing 10!)
As a cheapie option, you can use "greaseproof" paper or tracing paper. If I were using either of these I would probably bond it in acrylic resin on to a piece of 2mm acylic sheet.
You can make a lovely pearl style diffusion by pouring a layer of acrylic resin and adding a small amount of (white) mica powder.
Even with completely even diffusion, if the desired end result is an evenly lit panel, there can be a problem with light "hotspots". You can level this a little by using some direct diffusion on the leds. I've used all manner of things for this - a bit of hot glue, dab of slightly opaque resin or (best) a thin skin of Fimo puppen (white). You can "cook" it on the led. That's one way to make those cute little mushrooms, too.