i just finished designing another multi-cave (i suppose multi-terrace would be a more accurate term) and here's some photos. i'll update it with more photos after i put it my 20g.
the dimensions are approximately 12.5 x 5.5 x 7.5" (length x width x height), so it'll take up around 40% of the tank's length and 50% of the tank's width. i wish i had a bigger tank to put it in...
Nice! Couldnt you add some kind of plastic plant leaves and add them to the rock's so you have like a curtain they have to swim through, giving your fish privacy?