Like others have mentioned, I found that using Hot Glue to stick the Piezo Disc to the box, then putting a glob of Hot Glue on top of it really helps to get rid of any 'tinny' sound. It does, however, muffle the sound somewhat where the Amp Volume needs to be turned up a bit.
The biggest benefit that I see to the Hot Glue technique is that I got NO unwanted sound of my hand running up and down the CBG Neck, and NO sound from my strumming hand hitting the box.
Basically I found that a single piezo taped to the CigarBox was tinny and picked up the sound of my hand rubbing up-and-down the CBG neck. (Sounded like a hissing sound.)
In contrast, a single piezo 'hot-glued' to the CigarBox got rid of that tinny sound, and also got rid of that "hand-on-CBGneck hissing" sound. BUT....a lot of volume was gone! Also, lost that traditional CBG sound.
So I wired both piezos in parallel (one taped to the CigarBox and one hot-glued to the CigarBox) and got a great sounding CBG! The sound is 'full' and though the "hand-on-CBGneck hissing" sound is still present, it is 'masked' by the richer sound of the hot-glued piezo. I am pleased with this sound, as it seems to have gotten the 'best-of-both-worlds'.
(Side note: The best sound was by turning the Bass all the way up on the amp, and turn down the treble.)
Sounds like you may have created a humbucker.