If you don't want water splashing around in the reservoir the build some walls in the tank in the shape of a "#" sign but leave holes in the bottom for the coolant to move freely without sloshing in the reservoir. This is commonly used in gas tanks and aftermarket oilpans to prevent running dry when the vehicle is heavily cornering or braking.
Also if your going to use the tank for filling and bleeding then doesn't it have to be at the highest point in the system.