Gum shield recommendations?

I've been sparring reasonably regularly for quite a while now and feel it's about time to upgrade my gum shield. Currently I have a really cheap one I got off ebay - one of the ones you heat up and it's supposed to mold to your teeth. I've also hacked a lot of the back off it with an exacto knife to make it more comfortable and less inclined to hold lakes of spit.



So I'm looking for recommendations for a good gum shield and don't mind paying for quality (within reason) but I'm not really sure what sort of features are most beneficial.



I guess my priorities are, nice wide air channel, doesn't hold spit, comfortable.