I recently acquired the Extron IN1508 because I wanted to push my Atari 2600 clone (Sears Video Arcade II) on through the OSSC and I’ve only ever modded it for S-Video output. It’s got two composite and two s-video inputs along with a flurry of other higher quality connections that all route through a 15 pin d-sub connector that will plug in the OSSC’s AV3.
After some playing around with it to learn the setup I’ve been pretty pleased with the outcome. The nice thing is they can come fairly cheap on eBay. I only paid about 33 bucks for mine including shipping.