That adapter includes pointless leds and attempts to clean up the signal in various ways to make it more reliable. You can likely do without for a one off transfer, at the cost of some hassle getting your audio output right.

The 'simple stupid' approach is to cut off a 3.5mm cable, de-isolate one of the wires and tape it to the read pin. I've done that (just connected random analog signals straight to an io pin with tape) to interface to other ports on the c64. It's incredible what it will tolerate.

But sure, if you have both a cassette player and a datasette to hand, it will definitively be easier.

Haha, I wish I had known these things back in the day! :-D

Thanks, interesting.