Hipass/Lowpass Filter with Booster
www.musikding.de
(docs)
https://schalltechnik04.de/en/instructions/vong-filterung
And a simpler 12dB/oct HPF, which I think is the format Josh Broughton uses:
https://www.musikding.de/MINI-HP-VONG-filter-kit
And then Francis Deck's fdeck HPF units are open source and he's published the schematics.
I just finished up a board layout for an HPF/one band PEQ combination that will run on +/-15V for more headroom, using a DC-DC converter to allow operation on a standard 9V supply. The design has been well vetted in my DIY bass amps and should give legit studio quality even with the most demanding instruments, or be able to drive pro format PA amps directly. I'm open to barter and whatnot as long as I don't have to do a crazy amount of support beyond a BOM and schematic. But all my stuff requires intermediate level SMT soldering skills, FWIW. Nothing tiny, just mostly 1210, 1206, SOIC-8, and occasionally a few 0805 ceramic caps. All the critical caps are through hole, mostly polypro, and easy to find at Mouser or Digikey.
If you just want a standalone fixed 35Hz 4th order HPF it could be really small, something like two TL072 would easily get it done. You could probably even get by with just one if it's gonna be sandwiched between two other pedals, or driven by an active bass. I'll be happy to work up an LTspice model and share with the class since I already have all the building blocks worked up.