A DSB in an 18" tall tank? You should go open top, and build 1 or 2 taller islands that come out of the water. A simple aqua lifter plumbed strategically would supply enough water to keep the exposed rock saturated, and you could even make a tide pool lagoon in one part of the tank as well, your options are endless with a little creativity. Islands would be no problem, you will always get lower flow areas, regardless how you scape, so just do what you want to look good and correct flow issues with good placement of powerheads. This sounds like a really fun tank build, I look forward to seeing it completed.