First small rack? Don't you mean first small BIZZARO rack? MUA-HAH-HAH-HAAAAA!
OK, let's make it bizzaro...
Keep the MATHS where it is. I assume you're using a self-contained 84 hp rack, such as a Doepfer A-100LC3; this will have internal power with a built-in DC 'brick' input. If that's the case, this build should work.
Next, add the Batumi and the Poti expander. You will likely want the extra flexibility. So...ok, this gives us the Batumi for modulation, and the MATHS can do a lot of the envelop-ish and slewing behavior.
Then...BIZZARO oscillator! The Harvestman's Hertz Donut mk II should be perfect to compliment the analog oscillators in the two 0-Coasts. This is a very strange but extremely capable dual digital oscillator with internal modulation paths and TZFM capability. The spacing on this also takes care of the odd offset from the Poti, bringing us back into even hp turf. So, what better to go with a BIZZARO oscillator than...yeah, you guessed it...
BIZZARO filter! This thing is brilliant for strange timbres and filter modulations: the Limaflo Motomouth (https://www.modulargrid.net/e/other-unknown-limaflo-motomouth). It's a multiband formant filter based not on typical cutoff and resonance models, but emulating vocal vowel formants and morphing. Put together with the Hertz Donut, you then get into this very strange, alien voice-but-not-voice territory.
Next, not BIZZARO...just a dual VCA from Doepfer, their A-132-3. These VCAs can be switched between linear and exponential behavior, so while one can handle audio dynamics (f'rinstance), the other can deal with controlling a modulation signal's levels being fed...well, most anywhere. Since this is sort of a monosynth (ok, yes, a BIZZARO monosynth), there's really only a single signal path for audio at this point, so having two VCAs like this is pretty optimal.
Next...more BIZZARO! Right next to each other, a Synthrotek VERB, which is a CVable mono reverb, and a Feedback 106 Chorus, which not only modulates the timbre, but spreads the mono VERB signal into stereo in a few different, manually-controllable ways, so that its output goes right to a Malekko Output which gives you a ganged stereo level and steps your signal down to regular ol' line level. Not BIZZARO line level, mind you...just the usual.
Fills all 84 hp neatly. And there's way more than ample room for exploration there...plus it's going to sound NOTHING like the 0-Coasts or, really, most any other monosynth, because this is a...yep...BIZZARO monosynth! Also, you can patch the 0-Coasts in and out of this really nicely in oodles of ways. The SQ-1 could, say, sequence only one half of the Hertz Donut, leaving the other oscillator fixed to create lots of Conrad Schnitzler-esque clangers, or...well, you get the BIZZARO idea here, I presume.
(Last damn time I watch that 'Sealab 2021' episode before trying to get something done! And NO, not something BIZZARO done!)