Aha! I was working on adding new stuff and making corrections on EMW's listings, and the very thing needed here just happens to be one of their modules: https://www.modulargrid.net/e/emw-sub-osc-generator
The very thing, with 1 and 2 octave suboscillator, "tone" control (waveshaping, basically), fits in 6 hp and only costs $69 + shipping from Brazil. I would suggest going direct with EMW, also...their distribution can be spotty in some locations.
And oh, yeah...it's got its own 4-point passive mult to boot!