You could try one of those tuning inductors.
The ones from old car radios move a bit more than a centimetre,
if you fed a long wave one with a suitable frequency, you could
get a decent impedance change with the movement of the insert.
Or maybe one of those little "Led and Sensor" units which have an
Led and a sensor in a little plastic unit with a gap, like they used to
use on car distributors ... i reckon you could make some piece of
card or plastic to fit the gap, with maybe a slot that widens with
the movement you want.
Some simple potentiometers will handle the usage and life that you
propose, the type that are fitted to some motorcycles, i'm afraid i
don't know much about them other than they have extremely high
use-age handling.
I will try to find out what i can about those potentiometers,
i don't think they are very expensive.
John