With the pipe acting as a "wave guide", the range is much further than it would be in open air.

I think even the previous ideas of submerged capcitors etc. would cost at least $30 in man-hours alone building it.
You can mount a pipe in a few minutes.

And you could always "roll your own" with an ultrasonic sensor @ $5.00, if the $30 is too much, but then there's that time vs. money problem again ...
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