Fashion a lump of something heavy into a block with a flat, smooth face. Glue a piezo transducer to the block - this one is a cheap 20mm diameter type. I've drilled and tapped a couple of screw holes in the block for later...

Solder a length of screened wire to the piezo transducer, and clamp the wire for strain relief - the piezo element's metallisation is extremely fragile and easy to rip off.

Make a thin piece of something hard but electrically-insulating into a disc, and glue it on top of the piezo element. I used a spare piece of FR4 circuit board. Cover the exposed solder joint in tape.

Wrap in copper tape for shielding purposes. Ideally also solder the copper tape to the screen of the wire if possible.