A lot depends on what you want to use it for.
Mirosol made a
vero for a 2-channel mixer, which would probably work well for most applications with an appropriate switching scheme.
The active mixer would be overkill for simpler applications, though. For example if you simply want to mix the stereo output of your mp3 player so they can be played through a single speaker, then you should read
this article. You could make a very simple box with 3 resistors and a switch, no vero required. If you didn't need to switch between mono and stereo, it could easily be built into a cable.