3.5mm Stereo output to 3.5mm mic input crossover
If your recording device is like the RPI, a linux based system with usb inputs, Any generic usb sound card with a stereo/line level in would work. About 2 bucks on ebay with china shipping. Built-in support in 99% of Linux Distros.

If not, see Circuit to Convert Line Level to Mic Level for a way to convert Line Level (which most common outputs are) to Mic Level signals. A pair of resistors and maybe a cap. Keep in mind most Mic inputs on pcs and similar consumer hardware are MONO, not Stereo.

Also a premade Patch Cable (also called an Aux cable, check the dollar store) would likely work better than your handmade one.