I have a little plastic baggie I use. I don't make lots of noise with it, but at home they're fed their treats after work from a similar sounding bag, so if I dangle it in front of them in a class and move it up and down, voila! I try to only give one treat during the class, usually after the judge has viewed my horse and is almost done with the next horse. That way if the judge looks back my horse isn't munching, but the horse know if they keep at attention they WILL be rewarded. And the bag easily stuffs anywhere and isn't messy.
I've thought about trying a little pocket mirror...anyone ever tried this?