I would use a mic .
I had run across this a little while back, quick search and found it again, DIY clip on mic for harmonica in a rack
I have tried various mic attachments to my rack. I found them all to be a pain in live situations,swapping harmonicas .extra cables to get tangled in. What have done in the past is use a mic on a stand with a foam wind break on it rest your harmonica right on the mic . You want a fairly thick foam one or just double up thin ones. Just my 2 cents.
you might be interested in this ,here is a link on how to build an inexpensive harmonica mic.http://www.planetharmonica.com/ph2/VE/TMI-micUK.htm
image.jpegHey ,I've been.making hands free harp holders so I can play harp one handed since I lost my left hand years ago . I have one you mite find useful. I make them from aluminum stock and they screw onto a Shure 58. The slot has wiper blade that hold the harp and act as a seal.Here are some photos .image.jpeg
Just sent a post with some photos ,but the ones displayed aren't the ones I sent ? How to fix ? Ron
I'd do it the same way musician's have done it for decades. Small lightweight bracket around your neck that holds harmonica and has a clip on mic. A mic like that will work so much better than a piezo. You could use the C.B.Gitty Tin Can Mic if you want that old time echo effect.