Welcome to the PPCB forum, Ethan.
Check out DeadEndFX "TRESONATOR" — Korg MS-20 Filter in a Wah-shell.
Bill Laswell got some good dubby effects with a DOD 545 — I have the schematic, but know of no layout thus far…
Laswell's Trick:
— turn down the 545's sensitivity all the way down, the filter locks on to the highs leaving only the lows. When the sensitivity is rolled all the way down, the filter won't open and Range control has no affect. [WHAT IS RANGE? THE 545 HAS ATTACK AND PEAK]
The other thing you can do is roll the sensitivity back but just to the point where normal playing doesn't come close to opening the filter and really digging in gives that big pulse in the lows but doesn't open the filter all the way. You'll likely want a limiter/compressor but it can be a cool sound too. Laswell plays with flats, I think.
Lotsa people think he uses an FX25, for which you can
get the PCB here. You should still be able to get close to Laswell's sound with a FX25.
The 545 is an Optical-based filter, like the Mutrons. The FX25's are OTA based, AFAIK. A clean booster in front, such as the microamp, can help.
If you're new to DIY pedal-building, I'd highly recommend building a simple boost or fuzz to wet your feet. Low parts-count makes trouble-shooting easier and then build increasingly more complex builds 'til you're confident in your abilities to fab up a filter with the mods you want.
Some great EQ, compressor, and pre-amps for bass in the PPCB Shop/Forums, not to mention modulation, overdrives and others...