A system I've used for years for fixing piezo pickups to banjos- get a sliver of flat smooth hardwood veneer. Using thin double-sided tape, stick the veneer to the head under the bridge, so that the bridge rests on it and there is sufficient area behind the bridge onto which to tape the piezo pickup (the best tape is used by printers for sticking lithographic plates down- you have to find a printer and ask for a little bit). If everything is not securely taped to the head you get buzzes, but its a fine way to work otherwise. Don't forget it will raise the action slightly, so some bridge adjustment may be needed.