May I suggest you make your own rock? Sure, it'll take some effort and some time to cure (6-8 weeks), but in the end, it costs very little, it's fun, and you get exactly the shape you want.
For example, if all you want is something to serve as a base to support the pretty stuff, you can make a table with legs the height of your sand and then shape the table top to fit your LR perfectly so it won't rock or roll and tilt over. In other words, you can create a "seat" for the good stuff. This has the added attraction of leaving more cubic inches of space for the DSB, specifically the space in between the "legs" of the table.
If you make your "table legs" long enough to get your "table top" above the surface of the sand, you can also add various hidey holes in your D.I.Y. rock for critters and fish. The possibilities are endless. ( If I remember, I'll come back to this post when I get home tonight and edit it to add some pics of some D.I.Y. rock so you can see.)
Sure, it'll be a boring, monochromatic gray at first, but after a year in the tank, you'll never know it was D.I.Y. rock anyway.
