It's a lot easier just to make it up as you go along - and it also helps if you have a strong musette tuning on your accordion.

Something in a predominantly minor key with a few short chromatic runs, up and down octave jumps on the dominant (in A minor that would be E) and the occasional arpeggio - that would fool the troops!

Then there's 'Under the Bridges of Paris' and the theme from 'Maigret' which would probably be recognisable to a lot of people...

I wouldn't bother with the piano unless they really insist!