Back when my little shih tzu pupper was ready to be fixed, the vet tech gave me some great advice. Instead of buying a cone that would be too big for him or a donut he could get out of, she said to go to the dollar store and buy a baby onesie.
I took that advice and not only did it cost way less than something from the pet store, it meant that he could move around comfortably, not get at his stitches, and looked really darn cute.