If you're going the passive route, they're super easy to build. Five components will get you a passive volume control circuit: four 4k7 resistors and a 10K log potentiometer. Add in a few rotary switches for monitor selection/mute/mono/diff/etc and some XLR connectors and put it all in a box and you've got yourself a monitor controller. The caveat with a passive design over an active design is that changing volume does affect the impedance that the monitor or amplifier sees, which, depending on your amp or monitor, can affect the frequency response at low levels, especially with longer cable runs, hence why I went with an active design.
If you're really not comfortable with that though, there are plenty of options for passive, just be weary of the crazy price gouging that goes on with studio gear. Feel free to reach out if you need any help.