John, those turned out very nice! Too bad they interfere with each other. Wonder if some type of a trackside enclosure would help the interference, maybe narrow the sensing area or something. Still quite useable for many things though.
If you are planning to sell any of those PCBs, I would be interested in at least two of them. I have some of the HC-SR04s I got a couple years ago for something else and never used. And thanks for creating and testing!