just buy some wood and hardware cloth. You can easily build a frame with 2"x1/2" boards, or 2"x2" boards. Cut to fit either directly ontop of the tank, or inside, using the ledge most glass tanks have inside, as long as it is snug. the hardware cloth is sturdy enough that a cat can walk on it, but because of the gaps, it's possible the cat may find it uncomfortable on her claws (and maybe she will stay off ocmpletely). This is what I used for my 35gal tank I had my bearded dragon in till she outgrew it. Then I used it for my cornsnake (it was secure enough he never was able to excape it, even when he was 4' long).
Another good t hing by building your own lid, it costs a heck of a lot less than buying another of the tank covers at the store and being much more sturdier.