RCA / Cinch Y-adapter (stereo to mono downmix for subwoofer) by jk_printer | Download free STL model | Printables.com
RCA Y Cable - RCA Mono Male to Two RCA Stereo Female - Dedicated Audio
Y Adapter - Stereo 1/4“ TRS Plug to L/R mono RCA - 10-Foot Black Cable Cordial Essentials Series (3719851) by Hal Leonard
Amazon.com: C2G Legrand RCA Male to RCA Female Signal Splitter Cable, Black RCA to RCA Splitter, 6 Inch Audio Stereo Y-Cable, 26 AWG Audio Signal Converter Cable, 1 Count, C2G 03177 : Electronics
RCA Mono Male RCA to Two Female Stereo Adapter Y Cable - 6" - EACH - Dedicated Audio