This focussing effect is very strong with WAVELENGTHs less than the aperture of the reflector, making it particularly suitable for recording high frequency sounds such as birdsong. Schematic diagram ...

To me, this means a large parabolic reflector, a feed horn made of brass sheet, coat hanger wire, and at least for the initial experiments, an RTL-SDR dongle. I’ve done the calculations ...

The one component that may be hard to find is the parabolic reflector. You cannot simply use a bowl or other curved object as the precise parabolic shape ensures that sound waves are reflected ...