So I decided to try a few different materials in the search for an "ideal LCC plate. I won't bore you with all of the details, but will share a few things I discovered.
First off, the Capture Integration "2-stop" plate works extremely well, and I used it as my gold standard benchmark. I experimented with denser and less dense, or more accurately less transmissive and more transmissive materials.
What I learned is it didn't really matter much as long as you have a reasonably centered histogram on the capture file, the material has enough diffusion to give a smooth gradient to work with, and of course that it is neutral in color. Some specifics:
1) A denser material called "sign white" in 1/8th" thickness was a very clean white but required a 3-stop correction. While it worked very well, it did raise the noise floor on long exposures. I never noted that the noise corrupted an LCC, but it's a PITA to add 3-stops for the LCC when you're already shooting say a 1 second early morning landscape exposure. (In fact, I realized it's a PITA to have to change the exposure at all.) This material is also available in 1/16th inch, but it still required a near 3-stop correction factor, so not really any help.
2) I tried a 60% transmissive 1/8th" mild diffusion plexi that did NOT generate a good LCC. It did not require any exposure correction, histo was centered at base exposure, but it generated an LCC that over-correctied for lighting falloff by about 1/3rd stop. Not exactly sure why, but it did, and so the LCC was not accurate. The zero exposure offset was certainly welcome in the workflow, but since it didn't work perfectly it got tossed.
3) The material I settled on is called 40% transmissive "standard diffusion" plexi, again I used the 1/8th" thickness. This material has a nominal 1-stop offset, but the reality is the histo is so close to centered that you don't need to actually add the offset -- very convenient when shooting! This generated an identical LCC to the CI plate.
4) The other thing I experimented with was buffing to knock the gloss off so the surface was not reflective. Good news is it does kill reflections and generates an identical LCC to either my or CI's glossy plate. I have had a few issues when my CI plate was not firmly against the lens and the LCC had a weird reflection in it that ruined the LCC, so I decided this might help. My hope is this mitigates that problem and initial tests seem to confirm it will. I used a "green" Scotch-Bright synthetic steel wool pad to buff down the surface of the plate. (Note you can buy the 3-stop sign white material in matte surface, but I had already ruled out 3-stop material as unfriendly to my workflow.)
The only reason I started this search is the CI plate was not quite large enough to fit over my compendium hood and I was looking for a possibly easier workflow when using my tech cam with the hood than having to spin the hood out of the way. My larger plates worked great with the hood, but then I got another idea -- I modified the edges of one plate to slide into the filter slot of my compendium hood. For me this is an attractive convenience, and so far in use I like the hands-free bit
I obtained all of my material from a local TAP plastics store, but you can of course order online from them or probably find it in any well stocked home supply store.