There are several ways, but the most basic concept is to make a solid barrier.
Rocks work, but you need to 'mortar' them together. I use expanding foam. There is a black one sold for water falls, or the construction type that is usually yellow.
You could use expanding foam and no rocks. Just build it up slowly to the height you want.
A thin strip of plastic works. You would hold it in shape with rocks then silicone it at the bottom and where it touched the sided of the tank. Then fill carefully so the weight is the same on both sides. Do not just dump in all the gravel first- do a few scoops of gravel, then a few of sand, back and forth. This sort of material is sold in stores like Tap Plastics. There are several product names such as Lexan and others. They all work. Just find a thin enough sheet. To cut this material be VERY careful. I mark a line then use a razor knife guided by a straight edge. The cut does not have to be all the way through. Then snap it. If you are making a long piece it might take a couple of people to snap it. Silicone does not stick very well to these, so make sure all the oils from your fingers are cleaned off. It will hold long enough to build the set up.