While the light diffuser will keep jumpers in the tank, it won't keep heat in. Your heater will have to work longer and harder. Plastic (plexiglas) cut at Lowell's looks like a solution but will warp with warn tank temperatures. Been there done that. Glass cut to size, IMO
, is the answer. Good tip on Habitat store windows or tanks bought at 1$ per gal
sale and demoed. Also good tip on sanded edges on glass top piece. A good handle for the glass is one or two clear plastic, self-adhesive, table leg or chair leg bottom floor cushion. you can find them 1/4"-1/2" tall. Good luck.