The above mentioned thread is pretty much the only game in town.
If it helps: The alphabet used appears to be Devanagari script. That and Tibetan script could be used for quick text-block filler pages. I'd also suggest Alphabet of the Magi and "Bullskrit" fonts for extra variation and a nice Rami nod.
Unfortunately for replica enthusiasts (but fortunate for various spiritual traditions out there) it seems like they didn't really use any pre-existing imagery, and the heavy aging/burning/corruption of the pages means clear images are unlikely. I think we see a few more of the sigils in MOM, and a couple of somewhat clearer shots of the cover.
A source that might help with the geometric sigils/prism structures would be the Fullmetal Alchemist fandom (various "transmutation circles"), and of course a trip through free/printable Necronomicon pages for assortments of unpleasant-looking things to help bulk it up.
I'm planning on working on a "scale" Darkhold after I finish up my "scale" Rami Necronomicon - if I ever get a thread up feel free to lift ideas liberally. I love me a good movie prop book of terrible power.