You can cover the top of the tank but it may cause a slight increase in temp which would be ok. That would help a little. One of the benefits of a topless tank is the gas exchange that takes place. But with a sump you should be getting enough exchange.
You can expect to do 3-10 gallons a week top off. That would be my guess. I can do about 5g a week top off on my 75g that has a total water volume of 120g including sump and fuge.
It may look like a slow leak, but unless the floor is wet, it is just evaporation.