Can anyone direct me to a schematic for a basic stereo/mono switch?
The only guides I can find are to mix mono into the separate "stereo" channels.
I just need to switch between the two.

Ignore the "Lockout chip / switch"
Alternatively, you could use my Multi Use DIY Board to switch it with just a button press, which is much more reliable than DPDT :)
pots for the different channels helps too, unless you have a receiver with balance function to rather adjust instead.
-Segasonicfan