Oops! Didn't realize I'd made this rack public. LOL Honestly, you could use any other oscillator you'd like that would fit within the space. I don't remember what the HP is on the Primary Oscillator. A Befaco Even VCO would be a good fit and free up an additional 8HP. I was trying to find a way to recreate the Subharmonicon since it doesn't look like we'll ever see a production version.