Can

In the olden days of the 20th century, headphones were sometimes called: cans.  Well here is a 21st century version of high impedance cans (really can, a single element):

It uses the headband from a defunct Gemini headphone - the dynamic elements and padding were removed.  Cable ties were then used to attach the Altoids round tin lids (actual cans).  Then a piezo telephone sounder, a CPE-6080, was wired to a 3.5 mm jack.  Then a standard 3.5 mm cable can be used to connect to a radio.  I did try these with the Elecraft K2 headphone jack and they are usable although not super comfortable.  I could add another piezo to the other can in the future but for now this is it.

I also tried it with the old crystal set but no luck, the antennas currently in place are too small to be of use or some connection has broken over the winter - I'll try them again in the spring when antenna time returns.

High impedance ?  Yes, although there is a peaky response:

the CPE-6080 essentially looks like a .09 uF capacitor (max) to the rest of the circuit.  So no DC current will flow and there is a capacitive reactance of over 2000 ohms at 600 Hz:

source: https://www.omnicalculator.com/physics/capacitive-reactance 

Smaller piezos should have lower capacitance and thus higher reactance at higher frequencies - remember those "crystal" earphones  ?

Here is a link to the CPE-6080 datasheet:

https://www.sameskydevices.com/product/resource/cpe-6080.pdf

The CPE-6080 can be purchased on the Switch and Parts page (scroll down near the bottom):

http://wb9kzy.com/switch.htm

or a little cheaper from Digi-Key:

https://www.digikey.com/en/products/detail/same-sky-formerly-cui-devices/CPE-6080/2364641

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm