First off, terminology-wise, there's no such thing as a snaffle with shanks. Snaffles, by definition, don't have shanks. But from your description I'm guessing you're using something like a Tom Thumb. That's a bad bit. Ditch it.
Do you even need a bit for trick riding? Doesn't it just get in the way? I'm assuming your horse is trained to lope around the round path of the arena like most trick horses, right? And your reins are probably slack most of the time because you aren't holding them?
I would use a true snaffle (with no shanks) or switch to some kind of bitless option. Can you stop him in a rope halter? He's a broke, trained horse--I bet you could. It's safer in case the reins get loose and stepped on, too. Don't want him cutting his tongue off or flipping over on top of you. Or try some other bitless option.
If you absolutely must have a bit, I would personally recommend a Boucher of some sort. They are extremely stable in the horse's mouth as well as being fairly gentle. No shanks. It is a true snaffle for direct reining. The mouthpiece doesn't particularly matter if you're not using the bit much, but a two or three piece snaffle should work.