Generally the cat won't go after something it can't actually see, that's why they're so terrified and playful about boxes and holes.
I use hand cut lids made from acrylic sheet and a light on a frame longer than the tank. Most lids will be fine if you fill up the "gaps". Usually the best gap plug is silver tape.
You can even bend coat-hangers to produce outline shapes then tape them onto the lid to make hard covers for the holes on the back.
I prefer being able to see my fishez so I usually buy the glass lids with that plastic extension that you trim with scissors then use desk lamps when I set up a tank for someone. That way they've got light for working and can enjoy their fish from top and sides.
My cats use the tanks on the bottom rack to drink out of (I keep open top tanks)...great water bowl....lol......and its funny watching all the half grown Bettas go after their tongue as they lap the water and the cat going after the fish food when I feed....I think the cats think if they lap fast enough they can get the food...pretty funny none-the-less......I have never lost a fish due to cats that I know of at least......and I have a lot of cats and fish for that matter....lol.....