I've used something like this with success. It was years ago and I can't say if it was exactly that one, but you'll see several that look just like it and are probably identical. Don't be fooled by the "HDMI-to-RCA" "converters" that are simply a cable with an HDMI connector on one end and the three RCA connectors on the other. Those don't convert anything and won't work. It has to have a little box that actually does the converting.
Read the reviews for whichever one you pick and I'm sure people will mention whether of not it works with a Roku device.