No link, I got some 4-pin (5V, GND, TRIGGER, ECHO) units yesterday and got one working.
There's not only poor echo from soft (certain textures too?) things but for mine the 15 degree spread makes a difference.
The units I have have a thin metal screen in front that I think could be water/dirt soaked but you're not limited to using these units, in theory says the guy who hasn't made naked piezo disks work though that's what's inside the units I and most others have.
In theory you could seal the surface of a transmitter/receiver and it will work while putting a layer of wrap even a small distance beyond the surface should mess with the signal badly.
Also if you can hear it, what you hear is not the 40 kHz mine and most small units operate at. But that's not the only ultrasonic frequency used, I used to be able to hear store security and traffic signal units run at I was told around 23 kHz into the mid-70's (they're LOUD! must have driven dogs nuts!) so maybe you have lower-frequency units or you can hear amazingly high pitched sounds or your experiments caused secondary frequencies to be emitted.
I think that you can buy sealed transducers. I know that you can buy piezo disks. Making either suit your purposes isn't going to be dead simple but maybe not impossible either.
FWIW, I got these units I have with the same goal and already you've found a problem that I hadn't really thought of.