I was speaking to a gentleman a while ago and he was telling me his technique, which im going to try out pretty soon when I have empty tanks available.
Anyway he was telling me what he does first is spread a very thin layer of black silicone on the wall he is going to be designing. This will enable the foam to stick as it doesn’t stick well to glass.
He then uses insulating foam that comes in a spray bottle and expands. I’m not sure exactly what it’s called but im sure someone knows what it is and could be more specific. When this is dry you then take either a knife or what I think is better, a mini hand torch, and burn out the shape of the foam into what its going to look like. So you can burn out ledges and bumps and stuff.
Then when that’s done you lay it on its back and apply a crap load of black silicone on top of the foam. He told me its best to do it in small sections and mold the silicone to the shape you want. Then when you're done but it’s still wet you can sprinkle ecoearth/bedabeast on the silicone to give it a more natural look. Then leave it to dry and lift the tank up probably the next day so the excess falls down and presto, you have a sweet display tank.
I hope I explained that well
If you have questions, ill try to answer them more specifically. Good luck!