That's right, a plastic cup works fine as a mixing container, and I do both measuring and mixing in the same cup.  Marking a fresh cup only takes a minute.  The recommendation of a plastic spatula makes good sense.  I use a plastic stirring rod, since a spoon draws too much air into the mix for my liking, but a plastic spatula should work at least as well.  Metal tends to beat the cup up rather quickly, and the abrasive plaster itself will eventually sand the stirring rod, spatula, or other implement down slightly, so don't use something expensive.