This idea was inspired by Brad Newton's (aka FeatherfinFan) lava/cave background that he created for his tank.
Some crushed coral
Plastic rain gutter (90º corner piece)
Silk or plastic plants
100% Silicone glue
I went to home depot, and bought a corner piece for plastic gutters. There were two types, for inside corners and outside corners. I bought the inside type, which cost me about 10 dollars Canadian.
The first step was to heat the rain gutter over the stove so that I could shape the sides how I liked. Next, I smother the top and the sides with silicone. While it was wet, I glued on lots of pieces of crushed coral, which is what I use for my substrate. You could substitute this with whatever substrate you are using: gravel, crushed coral, or sand. Initially, the silicone did not hold that well so I then put some more on and added more crushed coral. I had a silk plant lying around, so I cut it up and stuck it into the silicone as well. I also made a small hole on top so I can put a real amazon sword in the hole. The piece was originally a corner 90 degree so there are two entrances to the cave.