I saw the Centrifugal Mug Carrier by Ecogeeco and decided to see if I could make my own. It's pretty much the same thing as the SpillNot I've seen from Arbor Scientific. It is sized to fit a standard coffee cup (83 mm diameter, 110 mm tall). You'll need to tie a string or strap to the ring.

You put a full cup of water or a mug of coffee and swing this around without any danger of spilling. Unless, of course, you hit someone with it. With a little practice you can swing this in big vertical circles or even in horizontal circles over your head.