Work fast when making the balls. Having the batch sit out while making the balls in combination with the heat from your hands will make it thaw out quicker and it becomes a lot harder to work with (I personally had to throw away two rice balls because it kept falling apart and wouldn't hold any shape). They are a little bland tasting (but that may just be because I used vegetable stock instead of chicken) so I recommend adding a few more flavoring agents if you so wish during the "risotto making" stage.
