There are all sorts of ways you could attach the piezos but its really important that they be held consistently.    Just taping them works ok but the tape slowly gives up a little and that can have big effects on the signal.    This is what I have found works the best.

Its a simple laser cut housing,  I used .25" acrylic but anything will work.  The housing is 1/2" deep and holds a 1" piece of  polyurethane that was also cut on the lase to the right diameter.  On the back side I have a simple screw terminal.

This is held onto the surface with 3M Command removable strips.   The setup keeps the piezo firmly to the surface and still allows it to vibrate.