I built my own cover for my 35 gal tank a couple years ago. I just used a 1"x1" length of board, cut it to fit within the indentation of a glass tank (the inner ledge of the black frame, so the lid sits snug on that lip). I made sure it fit snuggly so the snake can't push it out. I then stretched aluminum screen (same kind you use for windows) and stabled it tight with the stables close together (within an inch of eachother). Unfortunately I don't have any pics of the setup and I don't have it anymore.
I can honestly say my snake didn't escape from it but he was only about year and half when I sold him, and about 4' long. A larger snake may have been able to push off the lid but you can weight it down or attach clips along the sides to stop that.