Spray foam and rock.

[KevinK]

it is the black pond foam (kind of the great stuff) but black

I have it on my tank, and is gust great,

I made structures out of egcrate and sprayd them with it, I put one layer of live rock against it and within a month it was covered with coraline algea, so you dont se the differance any more between LR and foam

you can make caves, ledges and so much more, only dont forget to put it on a egcrate on the bottom, otherwise it will float up.

when you want to do your walls, also spray it on egcrate when on a newspaper ore so, and afterwards put it in your tank (best with euro bracing)

I can send you some pics from the build