Started 17th December 2018
I bought 60 transmitters from "Piezoelectric ceramic ultrasonic sensor TC40-16T/R transceiver 40KHz 16 mm split T and R". Following  I was surprised that these have undefined polarity. The pins on the back are polarised, but how the insides are connected to them varies. I connected a receive transducer to my 'scope and then connected each transmitter to a signal generator and sorted them based on the resulting trace. I 3D printed a tube (test spacer download below) to hold the two transducers a fixed distance apart. It was apparent that some transmitters produced more output than others, one produced no output at all, and I pulled it to pieces.
The original MidLev is designed to be printed in one big piece. I have split it into a number of parts - two bowls, held apart by two supports and standing on two feet. The three OpenSCAD files below render these three items, they're actually the same file with different parts commented out.
To make this work I have a small object under the middle of the bottom bowl. It would have been better to have continued a bit further and had four supports and feet.
Photos of my build using the above files.