I tried the pvc and silicone with substrate and it didn't work quite as easily as it sounds. Sand wouldn't stick to the silicone at all so it looks awful. I tried gravel and you have to take your time and put one granule on there at a time and give them time to wait for the silicone to dry before adding more. I've seen some diy on the web that look really nice but it's not quite as easy as it sounds since silicone doesn't instantly stick and takes forever to dry.
Least it didn't work out too well for me.
My next project I might try is similar. I went and got some large pvc and put it in the back corners of the tank. I think I'm going to just go grab some more large river rocks from the land scaping place and make a big pile of rocks balanced up in the corner and on the pvc, concealing the whitr pvc but leaving the pvc openings as a cave.