Step 1: What you will need
The silver disc you see is the lid off of a regular can of canned vegetables. Don't ask me what size. I don't know. It is about 3.5" in diameter. This will transfer the vibration to the piezo transducer.
The two black squares are rubber pieces cut from a mouse pad just larger than the can lid. Any flexible rubber or foam cushioning material will work here that is not to thick.
Lastly you will need a 1/4" instrument cable. you will cut one end off of the cable and strip the wires back to find the center conductor and the ground shield conductor.
A little duct tape and epoxy and you are there.
Update: After using the pedal for a while I was getting to many "double triggers" so I did a little more work on the sensor. What I ended up using in place of the mouse pad rubber was the thin cardboard from the back of a notepad (not corrugated). This seems to work very well and I do not get any double triggers now.