The grocery store where I would get my morning cup of coffee, changed the cup type to the cheap plain cardboard (and didn't offer those cardboard sleeves). I couldn't stand the heat! So I used the pattern on Knitty.com http://knitty.com/ISSUEwinter06/PATTslingshot.html
as a basic idea and just did my own thing (no sling). It works great!
I made it on one 2.25mm 16" circular, back and forth (slipping the 1st stitch every row) and sewed it up when done. Because I followed the stitch count in the pattern for casting on and any decreases, it fits nicely. And I just alternated knitting or purling for a group of rows (where ever I wanted) to add some hand grip. And using multicolored yarn helps disguise any spills until I want
to wash it.