You shouldn't have to refill it that often during the course of the year.
However, you could mod it in such a way that you wouldn't have to open up the case. What is the screw size of the fillcap at the top? One idea is to be able to put a barb sticking out of it and place a DangerDen fillport at the top of your case and run tubing from the barb in the fillcap to it. That way, all you would have to do is add liquid through the fillport.