For mine, I actually don't use suction cups, but I also don't have to separate that large of fish. For mine, I bury the bottom edge in the substrate with lots of rocks on either side, the sides and top have some foam pipe insulation on them that wedges them fairly securely to the top and sides. This wouldn't hold a big cichlid or something, but for small fish, it works great.
You can buy the suction cups for air lines. For a barrier I use eggcrate and 8 suction cups. Bury the bottom of the eggcrate about an inch into the gravel. You're going to use 4 suction cups on each side. That should do it. You might be able to buy suction cups from a acrylic or plastic store. The usually have different size cups and they're probably cheaper than the ones for air lines. The Doctor.
they were with the bathroom/kitchen stuff, you know things like shelves that stick to tub walls and little shelves that go over sinks. stuff like that.