I assume you mean how to build one? If so I would have to look. I just thought that one up.
I remember seeing some aquarists doing starboard with a gravel cover, on The Reef Tanks, advanced forum. I have also seen a few on RC and Reef sanctuary done in some form of a similar fashion. Several have poured some type of mixture right on the bottom, but thats not for me.
We need "Super Link Bev", here.
I,m bad with links but can try find you some if that what you're looking for.