Almost all of the built-in saturation effects in REAPER only add odd harmonics.
The only ones I found that are capable of adding even harmonics are the Bad Buss Mojo Waveshaper and the Graphical Waveshaper, although these are fairly difficult to control.
What simple, free options are there to add pleasing even harmonics? JSFX is preferred, since JSFX can be used in the Reaper Mixing Contest (
https://forum.cockos.com/showthread.php?t=271258). Thanks!