Inexpensive flat sensors which respond to contact pressure are available from:

Interlink Electronics
546 Flynn Road
Camarillo CA 93012
805-484-1331 voice
805-484-8989 fax
amadhav@interlinkelec.com sales email

The small ones are around $US3 in quantity. They also have a kit with
several types and some application info. If memory serves me, I paid about
$US75 for it last year.

I have no financial connection to them; this information is supplied just
as a matter of interest.

Reg Neale