Ready built simple detector
In the page Simple heterodyne / FD detector you can see different designs for detectors that uses quite simple technique.
I had a bunch of PCB's made for the frequency devider detectors and heterodyne (arduino) detectors.
I sell these as ready built detectors.
The FD uses very sensitive ultrasonic transducers and needs no display, the HET (heterodyne) uses a mems microphone element and a oled display
Choose the technique you like best!