Can I convert stereo to mono for a phono input

My preamp doesn't have a switch to convert stereo to mono.  I would like to switch a stereo signal to mono to set my cartridge azimuth (when using two channels out of phase, this method allows accurate balancing of the channels).  Is there a way to build a simple converter: two RCA female plugs taking a stereo signal and mixing it to mono, output as two RCA male plugs (mono signal)?

I have a test LP that provides a stereo track (test signal) with the two sides out of phase.  All I would need to do is feed that through the mono converter to set my azimuth. 

Thanks for any advice.  Peter