DIY Ultrasonic Microphone?

[rogs]

imho high sampling rate is only moderately important you still need a good sounding microphone. I've done this exercise with lab mics with impressive specs and the result is unusable from an artistic standpoint.

Depends on the type of ultrasonic recording you are undertaking.....
I can see the advantage of using a very high quality mic for recordings that are likely to be re-sampled or frequency shifted for 'artistic' purposes.
If however you are looking to record bat ultrasound transmissions, then other parameters - like using a 32 bit float format - can assume a higher priority.
The dynamic range encountered recording bat ultrasound - from essentially fast moving unpredictable sources - can be quite challenging, and removing the need for accurate gain setting in advance can be more important than the 'good sounding' properties of the microphone.
As with many different forms of accurate sound reproduction, there is a 'horses for courses' aspect when it comes to specific types of ultrasonic recording..