Why not buy an ultrasonic tapemeasure, I bought one recently for $15, and find another use. The one I have has a "test" pin inside at which the raw unmodulted signal is available, and as soon as I get a clear spot on my bench, I'll investigate further. I already have the test point extended to a socket mounted in the case. If you do need to build your own what equipment do already have ie transducers? as the transducers determine the operating frequency. For the two transducer approach try a Polaroid setup.