(The video here ( http://www.candlepowerforums.com/vb/showthread.php?312940-Improving-the-DIY-integrating-sphere ) demonstrates just how effective using barium sulphate is.)

FWIW, if you want to do indirect diffusion, 50/50 barium sulfate and white paint offers a good balance of reflectant and cheap.  https://www.triticeaecap.org/wp-content/uploads/2011/12/Barium_Sulfate.pdf

Besides Canal Street Plastics, what other stores in NYC do you like for wearables materials and supplies?

I used a sheet of fresnel lens bent it and used it to show a distorted light bar from some rectangular LED near its focal plane back in the old days.

Last year at Target I saw some amazing Christmas decorations that had LEDs behind sheets of "prismatic" plastics and I've been wondering what that material is called ever since. The plastic had the texture of a fresnel lens, but made stripes of rainbow like a diffraction grating. Anyone know what that stuff was? 

Lol.  I used fresnel lens back when I was in university ages ago.  I didn't see your post until now.

Do you have any design guidelines for getting uniform light dispersion? How many LED's per square inch vs materials. I would probably want the least LED's, so my material choice would be based of the highest even light vs number of LED's, but I don't know what is a good starting point of what works before you can see local bright spots.

I've used a length of "clear" aquarium hose, split lengthwise and snapped over a multi-LED PCB to make a glowing light bar for a racing drone.

https://www.instagram.com/p/8dYrHgBHlJ/?utm_source=ig_web_copy_link

What would be your recommendation for shinning an LED through the back of a rhinestone? I had been considering using E6000 or a white silicone adhesive to diffuse the light. 

Have you found any awesome off the shelf parts you can't wait to combine with LEDs but haven't had the chance yet?

I've been experimenting with pourable 2-part epoxies, pouring them into curved molds (silicone cupcake cups, etc) and then laying a NeoPixel ring on top of the liquid to create an attached lens that projects the ring's colors in specific ways. Right now it's all experimentation - I make a lens-ring combo, then I turn it on and place it facing a white sheet to see what kind of effect it has on the emitted patterns. 

Is it possible to use tools like blender or other 3D software to reduce the trial-and-error aspect of a process like this? Are there resources to get plastic optical parts made either to spec, or in generic 'standard sizes' that would fit over common led-ring PCBs?

I've done some looking for things like this. It seems alot of what people recommend, like vinyl for light boxes, doesn't have great light transmission stats.

These are the best ones that I've found, I've listed the transmission % and the pricing if available below. 

PLEXIGLAS® LED (truLED) White WH14 GT, 47%, 45,93 GBP / m²
PLEXIGLAS® LED Clear 0M200 SC, 58%, 83,35 GBP / m²
3M™ Envision™ Diffuser Film, 3635-70, 30%, 50%, 60%, ???
3M™ Envision™ Diffuser Film, 3735-60, 30%, 50%, 60%, ??? 

Sadly, I won't be available right when it's going on, however, I did want to share our experiences at Unit-E making arcade games.  For really uniform lighting we found that tubes, sheets and chunks of LDPE couldn't be beaten.  Using sanded acrylic produces hot spots, UHMW gives a yellow tinge, HDEP is too opaque.  LDPE is just the most amazing, and easily machinable plastic for this sort of thing.  Except for edge lighting, then yes, milled or laser etched acrylic.  But I am really surprised it feels like LDPE is really overlooked in the maker scene...

An example shot of one of our machines: http://gaming.moe/wp/wp-content/uploads/2018/03/Unit-e_NeonFM.jpg

Hmm LDPE in sizes larger than 24x36 seems to have a yellowish tinge, at least from Mcmaster Carr. 

Do you have a thickness you recommend?

For whatever it's worth, and I would wager many folks have already tried this one - but I've been using fairly generic office supply "Backlight Film" to get nice soft illumination from LED's. It's also handy to run through a printer for a backlit gauge scale (or similar). Its designed for backlit signage. 

This blog is a great idea.  I'm posting here mainly to effectively subscribe to whatever anyone says about the diffusing light.  :-)

Personally, passing light through materials has been something I have preferred to avoid if I have the option of reflecting off of a surface like indirect lighting.  Whenever possible I prefer the indirect approach.  If the reflective surface is bright enough, there is minimal loss, but that solution does take some serious geometric creativity, but I'm sure the maker community is up to that. :-)