I can't say enough about making sure the cup has a wide enough mouth for ease of cleaning and putting in powder.
With a Nalgene bottle, it's almost impossible to scoop powder into the narrow mouth without spilling some of the precious stuff. I'm always either using a funnel or putting the powder into a separate paper cup that I can pinch to form a pouring spout.
The other shakers with adequate mouth widths (and shaker screens) unfortunately never screw shut as well as the Nalgene bottles. I've had a couple of shaker spills in my backpack because of faulty seals (pretty messy).
The perfect shaker would be a Nalgene-like bottle with a 4-5 inch diameter mouth, capable of holding about 40 oz of liquid. If somebody knows of such a bottle, please share.