I was thinking ultrasonic range finder applied in the Z direction. Sort of spendy from Sparkfun but should be available locally (== quickly). Online I've seen them for as little as $5 for something of reasonable quality.
Make sure you mind the minimum ranging distance spec., if any. Not all of them work to zero but then you don't want it getting wet anyway so mount accordingly. I have no RPi experience so timing the ranging pulse is left as an exercise. :-) Regards, NealS
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