I am currently working on some high capacity memory. Currently its 28 blocks per bit in volume for the storage, i have not finished the decoder/encoder yet.
This memory is based around items representing Hex digits, so i can technically fit a 20 bit number in the current storage with only changes to the encoder/decoder. (Might be as high as a 40 bit number although this is getting a bit crazy.) The density can be even higher of you expand the base, which only changes the side of the encoder/decoder not the storage itself.
Also i have not looked at access time at all yet and i am sure when i do i will be able to speed it up after my Proof of Concept.
The build so far is on my plot. I will u0load image once I have the PoC working.