I use packing tape (similar to duct tape), and I do it "double sided" so there is no sticky side exposed. Take the cover plastic strip off, make sure it is thoroughly dry both sides, and then tape the opening on both sides so the two sticky sides where they are over the opening will stick together. Eventually the mositure will cause tape to lose its hold, but in my experience that takes months.
It doesn't take much of an opening for hatchets, especially at the small size of new fish. I was having them get caught between the cover glass and the tank brace, something I would never have imagined, but I would find the dessicated corpse in the morning. I moved them into the 70g which doesn't have a brace.
Floating plants like Water Sprite can help in this, because you can have the plant under the openings, and the fish won't jump through a plant. They only jump through open water surface. Lowering the water level helps too. I did this while I was trying to figure out how to solve the problem I had, and finally decided just to move the fish to another tank.
If they do jump out, it is possible the dog eats them. Cats will too. Before you find them.