How about finding anything that would make a "hill" such as weighted styrofoam, then glue some of your gravel to it to cover it? I'm a newbie to aquariums, so I don't know if that would work, but it's the only thing I could think of.
BTW, nice tank! If I were you, I'd put some sort of dark substrate in there. I think it would look nicer with something more the same color as the filter ~ would make the filter "blend in" more.
~ Linda ~
How many watts do I need to make my swimming pool a high light tank?