A rack blanking panel is a common item in racks done primarily for air flow management, but it also can be done for aesthetics. They are a black metal plates rack width and a given height such that when you have a product inside it you just see metal and the product.
The DS1515+ is a bit over 5.5 rack units high, so the plate would be rack width and probably 5.75 or 6 Rack Units tall with an opening in the middle for the NAS.