I have no answers but if you don't mind I'm commenting so I can see the replies. No doubt it will be helpful for me too.
Hope that you get the answers you need.
There's a couple of groups for piezo's that would be of help. Click the "Posts" tab at the top of the page, then click on "Groups". There's one group with just wiring diagrams.
There is a video from Ben somewhere on here where he adds a 0.01uF ceramic disk capacitor bridging the two terminals of the output jack plug - linking the red and black wires on Wayfinder's diagram. The result is a much rounder, less overdriven tone and a big reduction in feedback.
I know it took me ages to find the video, and when I downloaded it, I changed it's name, thus losing any link to where it is here, but it really is that simple. Take any piezo wiring diagram from the CBG wiring & electronics group here, and use the capacitor between the two terminals of the jack socket
Thank you everyone for your advice. It is very helpful to a first time builder. P.S. I did mean tone control.