Recently I bought a nice eurorack DIY kit and soldered my own polarizing CV mixer. The DIY kit is a really nice product from Synovatron, check it out:
The circuit I built is from Din Sync:
Building, soldering, finding components is really easy if you’ve got some soldering experience and know your way around electronics a little.
The kit comes with knobs, jacks, wires, panel. For the CV mixer all you need to add is some wires, a few 100K resistors and one dual operational amplifier IC, such as a TL072. For example this one from Mouser. From what I understand almost any 8 pin dual op amp IC will do.
I made the circuit on a breadboard and transferred it identically to the DIY board, soldering similar “rows” as the breadboard on the back side of the board by bending the component legs and wires.
One word of advice: you really should use a power supply to test it out. You do not want to damage your expensive eurorack gear. Short circuit errors are easy to make. The power supply needs to be able to give you +12 and -12. This is the one I’m using: UTP3702.