I was thinking about this the other day and I'm not sure if this would work but maybe it's worth a shot.

What if you took 4 - 2 or 3" wide strips of plexiglas, long enough to go around all 4 sides of the tank, and glued them on the ends to make a big plexiglas square that just sits on the top edge of the glass? From what I've noticed, fish usually jump the tank by jumping up right next to the inside of the wall, not 3 inches towards the center, so if anything jumps up there next to the glass they would hit the plexiglas wall and be deflected. Just a thought, I was going to try this but found that evaporation was too fast with no top so I had to get a full top for mine.