You might put some velcro tape on the edges (the fuzzy part) to take up some of the space. Depending on exactly how it fits on the tank, that usually takes up some of the gap without making it impossible to open or close.
Baby corns aren't big enough to get out off most tanks unless they can get their heads over the edge, so just keeping it a bit more snug should solve the problem. You probably could get some big elastic bands to put around the ends too, just in case... but that's a pain to get on and off without spilling water.
Of course, I recommend the metal-framed sliding tops that we make, but use what you've got!