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c00ps
My method is a little niche as my starting point is a software defined radio that I happened to have knocking about (https://www.sdrplay.com/rsp1a/)
Despite the fact that RF and audio are different things, essetially a microphone does the same job as an antenna...i.e vibrating and converting those vibrations into electrical current.
This means I was just able to buy some mini electret microphones, knock up a simple circuit to power one (after frying a few) via the bias-T from the radio unit and 3d print a parabolic reflector and basically just stick all the parts together :D
Haven't tested it on bats yet but I've successfully tested it by pointing it at my ultrasonic cleaner that gives off harmonic audio at 5oKhz which is roughly the right ballpark for a bat.
Will see if it actually works in the Spring! Lots of bats around here...