Maybe take a look at Bastl Propust for a compact (and diy) ffb. It’s more basic than the make noise but might get you places (not sure how they implement the different slopes on the inputs of the fxdf). You could experiment with different cap values too if you’re wanna get your hands dirty.
There are some old moog schematics around of low and high pass ffbs (passive, so non-resonant) and they are surprisingly simple.
I just started exploring this with my serge reseq but it’s fun to set the upper and lower settings (which are different bands) and treat them like left/right outs for various feedback stuff (or mid/side wackiness though this is still a bit of a black box to me in practice). I imagine it might be fun to make two side by side passive ffbs with slightly different component values for a somewhat similar effect (though the resonance in the reseq seems to do a lot of the heavy lifting regarding that feedback magic)
Also if you’re handy with a soldering iron consider L-1’s quad resonant EQ. It’s not a cheap build (but MUCH cheaper to diy than buy the “factory” version) but also a pretty easy one (most components are presoldered if I remember). It’s also an hp hog- I kinda wish it was only dual.
Also wanna give a shout of to the humble music thing simple eq. I was having some fun the other night putting the tilt half in a runway feedback patch and using that to tame it, let it run wild, tame it again, and so on… 