I'd go to the hardware store, and buy a bunch of different sizes, and see what works best for you.
Here's the sizes from McMaster Carr that I use on a regular basis for deer, elk, crow, duck, etc.......012, 013, 906, 907. Look for neoprene. Some folks use Buna-o's, but they are not as durable over time, or as flexible/stretchy.
Parker