There's a trick that evens the response of a pickup that was developed for use with a B-band, but I've used it successfully with other piezos, including separate element, continuous strip, or piezo rope types.
Get some self-hardening clay from a craft or hobby store. You don't need much. Make a thin strip. I usually roll out some with a rolling pin and cut thin strips with an Xacto knife.
Put a thin strip of clay between the pickup and saddle. Tighten the strings to force the clay in, and wait a day or two.
This has worked for me. In difficult cases I've put the clay on below and above the piezo. I also take the saddle down to compensate for the thickness of the clay, although if done properly, the clay adds little to no thickness.
And, most importantly, clean the rolling pin before the clay dries. (Don't ask me how I know this...my wife can tell you.)
The clay is removable. If unsuccessful, the saddle and pickup can be removed, the clay can be picked out, and the remainder can be removed with a damp cloth.