I built one of these this week. I used some 1/4 watt cheap resistors, 20awg wire, heat shrink, 3.5mm plug, and RCA plug. Put this on the output of the echo dot.

3.5mm plug https://www.amazon.com/dp/B01LWK3G1L/ref=cm_sw_r_em_apa_i_DO5ODbQM5TT7M

RCA solder plug https://www.amazon.com/dp/B07H9ZR2GD/ref=cm_sw_r_em_apa_i_HQ5ODbJPPQQPG

Resistor pack https://www.amazon.com/dp/B0792M83JH/ref=cm_sw_r_em_apa_i_-R5ODbZ7J7G0Y