This is tricky with filters along the ends of a tank (if I am seeing the photo correctly in post #1). If you can turn the filter to hang on the back, most any cover will work. There are the ones designed for the size of tank, usually with a light, and they have a strip of plastic along the back that can be cut out for filters and heaters. The least expensive is probably the glass cover sets that are usually two pieces of glass with a hinge, and the same type of plastic strip along the back. These sit down on the lip around the inside of the tank frame. The light then sits across the frame itself (or as you have it).
This does happen; even fish not that prone to jumping can do it particularly during darkness. Cories, loaches, tetras...any fish. Fish naturally jump when startled by something, maybe another fish under them, or things like the slamming of a door even outside can startle them. Except in rare cases, tanks should always have a cover. You need one under the light always (glass) as water splashed by a fish feeding can cause the light to explode. And there is evaporation, which is a lot more than one might think because the tank water is warm, and all this moisture gets into the structure of the room, much like a bathroom during a shower.
