I think that the foam tape method is still the best.......you basically want the vibration transmitted through the cone to the piezo, and, as the output is a waveform, polarity is not crucial.......it will still trigger. The only time you want a lot of free play in the piezo is if you are using it as a speaker, otherwise you can fracture the ceramic.
♫♫♫....I'D RATHER LAUGH WITH THE SINNERS THAN CRY WITH THE SAINTS....♫♫♫........THE SINNERS ARE MUCH MORE FUN :D ......♫♫♫
--- BILLY JOEL---
TD-12, 14" DIY SNARE, 20" DIY RIDE, 2 x 12" DIY TOMS, 2 x 10" DIY TOMS, 3 x CY-12R/C, PD-6, CY-12H, FD-7, GIBRALTAR THRONE & DBL PEDAL, YAMAHA 12 CH 1000W POWERED MIXER w 2 15"/HORN SPKRS, ACE-TONE 100W AMP FOR 12" SUB.
A-KIT: 11 PIECE PEARL SESSION SBX WITH ELIMINATOR PEDALS AND SABIAN/ZILDJIAN CYMBALS.